Semiconductor device

ABSTRACT

A programmable analog device in which data can be held even when supply of a power supply potential is stopped. The programmable circuit includes unit cells connected in parallel or in series, and each of the unit cells includes an analog element. A conduction state of each of the unit cells is changed between an on state and an off state. Each of the unit cells includes, as a switch of the unit cell, a first transistor having a sufficiently low off-state current and a second transistor, a gate electrode of the second transistor being electrically connected to a source or drain electrode of the first transistor. The conduction state of the unit cell is controlled with a potential of the gate electrode of the second transistor, which can be kept even when no power is supplied thanks to the low off-state current of the first transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification relates to semiconductor devices.

Note that a semiconductor device in this specification and the like refers to any devices that can function by utilizing semiconductor characteristics, and semiconductor elements, electro-optical devices, storage devices, signal processing units, semiconductor circuits, and electronic devices are all semiconductor devices.

2. Description of the Related Art

The configurations of semiconductor integrated circuits typified by LSIs are fixed at the time of manufacture and cannot be generally changed after manufacture. In contrast, in semiconductor integrated circuits called programmable logic devices (PLDs), considering a logic unit composed of a plurality of logic circuits as a unit, logic units are electrically connected through wirings. In a programmable logic device, the circuit configuration of each logic unit can be electrically controlled.

As above, the design of a programmable logic device can be changed after manufacture; thus, the use of a programmable logic device can drastically reduce the time and costs needed to design and develop semiconductor integrated circuits.

There are some programmable logic devices called complex PLD (CPLD) and field programmable gate array (FPGA). In either device, the circuit configuration of each logic unit is controlled by a programmable switch that is switched in accordance with data (configuration data) stored in a memory unit provided in the logic unit. In other words, programming data into each programmable switch makes it possible to change the circuit configuration of a programmable logic device.

Volatile memory such as SRAM is used as the memory unit. On the other hand, for example, Patent Document 1 discloses a technique for using nonvolatile memory composed of a floating gate transistor, such as flash memory, as the memory unit.

In addition, a programmable analog circuit having an array composed of functional units including analog elements is being developed. For example, Patent Document 2 discloses a programmable capacitor array in which capacitors, which are analog elements, are connected in parallel so that the total capacitance value of the programmable capacitor array can be changed.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2004-15060 -   [Patent Document 2] Japanese Published Patent Application No.     H9-27732

SUMMARY OF THE INVENTION

In recent years, the reduction in power consumption of electronic devices is an important issue, and lower power consumption of semiconductor integrated circuits used in electronic devices is also strongly required. In view of the above, in order to reduce power consumption, a driving method is proposed by which supply of a power supply potential to the entire semiconductor device or part of the semiconductor device is temporarily stopped and supply of the power supply potential is selected only when needed in a circuit unit that needs the supply.

When a memory unit in a programmable switch of a programmable logic device is made nonvolatile by using a floating gate, data stored in a circuit unit is held even when supply of the power supply potential is temporarily stopped. However, since electrons are injected into a floating gate to write data, there are problems in that a high potential is required and it takes a long time to write data. Moreover, tunneling current generated in data writing causes deterioration of a gate insulating layer of a floating gate. Further, a floating gate transistor has low data write/read speed and is thus unsuitable for intermittent operation.

Although a programmable circuit including an analog circuit (analog element) has not yet been widely used as compared to a programmable logic device including a digital circuit, development of the programmable circuit has been desired for circuit space-saving, a reduction in cost, and an increase in design flexibility.

In view of the above problems, an object of an embodiment of the present invention is to provide a programmable analog device with which data can be held even while supply of a power supply potential is stopped and with which power consumption can be reduced.

According to an embodiment of the disclosed invention, in a programmable circuit including a plurality of unit cells each including an analog element, the conduction state (on state or off state) is controlled in each unit cell. Each of the unit cells includes, as a switch of the unit cell, a first transistor having a sufficiently low off-state current and a second transistor the gate electrode of which is electrically connected to a source or drain electrode of the first transistor. The conduction state of the unit cell is controlled with a potential of the gate electrode of the second transistor. More specifically, the following configurations can be employed, for example.

An embodiment of the present invention is a semiconductor device including a programmable circuit, the programmable circuit including a plurality of unit cells connected in parallel, a plurality of bit lines electrically connected to the plurality of unit cells, a plurality of word lines electrically connected to the plurality of unit cells, an input signal line, and an output signal line. Each of the plurality of unit cells includes a first transistor including a first gate electrode, a first source electrode, a first drain electrode, and an oxide semiconductor; a second transistor including a second gate electrode, a second source electrode, and a second drain electrode; and an analog element. In the programmable circuit, the word line is electrically connected to the first gate electrode; one of the first source electrode and the first drain electrode is electrically connected to the bit line; the other of the first source electrode and the first drain electrode is electrically connected to the second gate electrode; one of electrodes of the analog element is electrically connected to the input signal line; the other of the electrodes of the analog element is electrically connected to one of the second source electrode and the second drain electrode; the other of the second source electrode and the second drain electrode is electrically connected to the output signal line; and the conduction state of the each of the unit cells is controlled with a potential applied to the second gate electrode, whereby an analog value output from the programmable circuit is changed.

Another embodiment of the present invention is a semiconductor device including a programmable circuit, the programmable circuit including a plurality of unit cells connected in a matrix, a plurality of bit lines electrically connected to the plurality of unit cells, a plurality of word lines electrically connected to the plurality of unit cells, an input signal line, and an output signal line. Each of the plurality of unit cells includes a first transistor including a first gate electrode, a first source electrode, a first drain electrode, and an oxide semiconductor; a second transistor including a second gate electrode, a second source electrode, and a second drain electrode; and an analog element. In the programmable circuit, the word line is electrically connected to the first gate electrode; one of the first source electrode and the first drain electrode is electrically connected to the bit line; the other of the first source electrode and the first drain electrode is electrically connected to the second gate electrode; one of electrodes of the analog element is electrically connected to the input signal line; the other of the electrodes of the analog element is electrically connected to one of the second source electrode and the second drain electrode; the other of the second source electrode and the second drain electrode is electrically connected to the output signal line; and the conduction state of the each of the unit cells is controlled with a potential applied to the second gate electrode, whereby an analog value output from the programmable circuit is changed.

In any of the above semiconductor devices, the analog element may be a capacitor, a resistor, or a photoelectric conversion element.

In any of the above semiconductor devices, the analog element preferably includes an oxide semiconductor.

Each of the unit cells included in the programmable circuit includes, as a switch of the unit cell, a first transistor having a sufficiently low off-state current and a second transistor the gate electrode of which is electrically connected to a source or drain electrode of the first transistor. With such a structure, data for changing the conduction state of the unit cell, which is stored in a gate electrode of the second transistor, can be held for a long period while supply of a power supply potential is stopped. Therefore, supply of a power supply potential to a part or the whole of the programmable circuit can be temporarily stopped, and power supply potential can be supplied only when needed, leading to a reduction in power consumption of the semiconductor device.

Furthermore, a plurality of unit cells are arranged in the programmable circuit, and the conduction state is changed between an on state and an off state in each unit cell, so that the analog value of the whole programmable circuit can be changed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are circuit diagrams each illustrating a semiconductor device which is an embodiment of the present invention;

FIGS. 2A and 2B are circuit diagrams each illustrating a semiconductor device which is an embodiment of the present invention;

FIGS. 3A and 3B are circuit diagrams each illustrating a semiconductor device which is an embodiment of the present invention;

FIGS. 4A and 4B are circuit diagrams each illustrating a semiconductor device which is an embodiment of the present invention;

FIG. 5 is a circuit diagram illustrating a semiconductor device which is an embodiment of the present invention;

FIGS. 6A to 6D illustrate steps of forming a semiconductor device which is an embodiment of the present invention;

FIGS. 7A and 7B illustrate steps of forming the semiconductor device which is an embodiment of the present invention;

FIGS. 8A to 8C illustrate steps of forming the semiconductor device which is an embodiment of the present invention;

FIGS. 9A and 9B illustrate steps of forming the semiconductor device which is an embodiment of the present invention;

FIG. 10 is a block diagram of a portable electronic device;

FIG. 11 is a block diagram of an e-book reader;

FIGS. 12A to 12E illustrate structures of oxide materials;

FIGS. 13A to 13C illustrate structures of oxide materials;

FIGS. 14A to 14C illustrate structures of oxide materials;

FIGS. 15A and 15B illustrate structures of oxide materials;

FIG. 16 is a graph showing gate voltage dependence of mobility obtained by calculation;

FIGS. 17A to 17C are graphs showing gate voltage dependence of drain current and mobility obtained by calculation;

FIGS. 18A to 18C are graphs showing gate voltage dependence of drain current and mobility obtained by calculation;

FIGS. 19A to 19C are graphs showing gate voltage dependence of drain current and mobility obtained by calculation;

FIGS. 20A and 20B are a top view and a cross-sectional view of transistors used in calculation;

FIGS. 21A to 21C are graphs each showing characteristics of a transistor including an oxide semiconductor film;

FIG. 22 shows XRD spectra of Sample A and Sample B;

FIG. 23 is a graph showing a relation between off-state current of a transistor and substrate temperature in measurement;

FIG. 24 shows V_(g) dependence of I_(d) and field-effect mobility;

FIG. 25A shows a relation between threshold voltage and substrate temperature, and FIG. 25B shows a relation between field-effect mobility and substrate temperature; and

FIG. 26A is a plan view of a transistor used for measurement and FIG. 26B is a cross-sectional view of the transistor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the present invention is not limited to the following description, and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be limited to the descriptions of the embodiments below.

Note that in this specification and the like, functions of the “source” and “drain” may be replaced with each other when a transistor of opposite polarity is employed or when the direction of a current flow changes in a circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification and the like.

In this specification and the like, the term “electrically connected” includes the case where components are connected through an object having any electric function. There is no particular limitation on an object having any electric function as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an object having any electric function are a switching element such as a transistor, a resistor, an inductor, a capacitor (also referred to as condenser), and an element with a variety of functions as well as an electrode and a wiring.

The position, size, range, or the like of each component illustrated in drawings and the like is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like disclosed in the drawings and the like.

Note that the ordinal numbers such as “first” and “second” in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. In addition, the ordinal numbers in this specification and the like do not denote particular names that specify the present invention. Moreover, these ordinal numbers are used in order to avoid confusion among components, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a circuit configuration of a semiconductor device according to an embodiment of the disclosed invention will be described with reference to FIGS. 1A and 1B and FIGS. 2A and 2B.

<Programmable Circuit 1>

FIG. 1A illustrates a configuration of a programmable circuit 330 in this embodiment. The programmable circuit 330 includes a plurality of unit cells connected in parallel between an input signal line IN and an output signal line OUT. In FIG. 1A, a unit cell 320 a and a unit cell 320 b are connected in parallel. Note that an embodiment of the present invention is not limited to this configuration, and three or more unit cells may be connected in parallel.

The unit cells 320 (unit cell 320 a and unit cell 320 b) are electrically connected to bit lines B (bit line B_1 and bit line B_2) and word lines W (word line W_1 and word line W_2). In FIG. 1A, the unit cell 320 a is electrically connected to the word line W_1 and the bit line B_1, and the unit cell 320 b is electrically connected to the word line W_2 and the bit line B_2.

Each of the unit cells 320 includes a first transistor 340, a second transistor 350, and an analog element 310. A gate electrode of the first transistor 340 in the unit cell 320 is electrically connected to the word line W, one of source and drain electrodes of the first transistor 340 is electrically connected to the bit line B, and the other of the source and drain electrodes of the first transistor 340 is electrically connected to a gate electrode of the second transistor 350. One of electrodes of the analog element 310 is electrically connected to the input signal line IN, the other of the electrodes of the analog element 310 is electrically connected to one of source and drain electrodes of the second transistor 350, and the other of the source and drain electrodes of the second transistor 350 is electrically connected to the output signal line OUT.

Examples of the analog element 310 which can be applied to the unit cell 320 include a resistor, a capacitor, a coil, and a photoelectric conversion element.

A potential supplied to the analog element 310 via the input signal line IN is selectively output to the output signal line OUT after having gone through the analog element 310 when the second transistor 350 is on. Accordingly, output of an analog value from the analog element 310 in each of the unit cells 320 can be controlled with a potential applied to the gate electrode of the second transistor 350. An analog value output from the programmable circuit 330 via the output signal line OUT is a resultant total output of the unit cells 320 included in the programmable circuit 330, which means that the analog value of the programmable circuit 330 can be changed by control of output from the unit cells 320.

Here, a transistor having an extremely low off-state current is used as the first transistor 340. For example, a transistor for which a wide-bandgap semiconductor such as an oxide semiconductor is used can be employed. With the use of such a transistor having an extremely low off-state current as the first transistor 340, when the first transistor 340 is off, a potential of the gate electrode of the second transistor 350 can be held for a very long period.

Note that there is no particular limitation on a semiconductor material used for the second transistor 350. In terms of increasing the speed of outputting a potential of an analog element, a transistor with high switching speed such as a transistor including single crystal silicon is preferably used, for example. Alternatively, the second transistor 350 as well as the first transistor 340 may include a wide-bandgap semiconductor such as an oxide semiconductor.

The description of data input/output operations of the programmable circuit 330 illustrated in FIG. 1A is given below.

First, the potential of the word line W is set to a potential at which the first transistor 340 is turned on, so that the first transistor 340 is turned on. Thus, the potential of the bit line B is applied to the gate electrode of the second transistor 350. Next, a potential is applied to the analog element via the input signal line IN. When the potential applied to the gate electrode of the second transistor 350 is higher than or equal to the threshold voltage of the second transistor 350, the second transistor 350 is turned on; thus, the potential applied to the analog element 310 is output to the output signal line OUT. For example, in the case where the second transistor 350 is an n-channel transistor, when the potential of the bit line B is high (e.g., VDD), the second transistor 350 is turned on; thus, the potential of the analog element 310 is output to the output signal line OUT. In contrast, when the potential of the bit line B is low (e.g., GND), the second transistor 350 is turned off; thus, the potential of the analog element 310 is not output to the output signal line OUT, so that the unit cell 320 is off. Note that the second transistor 350 may be a p-channel transistor.

After the potential of the bit line B is stored in the gate electrode of the second transistor 350, in a state where the potential of the bit line B is kept, the potential of the word line W is set to a potential at which the first transistor 340 is turned off, so that the first transistor 340 is turned off. Thus, the potential applied to the gate electrode of the second transistor 350 can be held without supply of a new potential. Since the first transistor 340 includes a wide-bandgap semiconductor such as an oxide semiconductor and thus has an extremely low off-state current, the given potential applied to the gate electrode of the second transistor 350 can be held for a very long period. Therefore, the conduction state (a state where the potential applied to the analog element 310 is output to the output signal line OUT and a state where the potential is not output) of the unit cell 320 can be maintained without supply of power supply potential.

As described above, the unit cell 320 which is a component of the programmable circuit 330 includes, as a switch of the unit cell 320, the first transistor having a sufficiently low off-state current (e.g., a transistor including a wide-bandgap semiconductor such as an oxide semiconductor) and the second transistor the gate electrode of which is electrically connected to the source or drain electrode of the first transistor. The conduction state of the unit cell is controlled with a potential of the gate electrode of the second transistor. With such a structure, data for changing the conduction state of the unit cell 320, which is stored in the gate electrode of the second transistor, can be held for a long period while supply of a power supply potential is stopped. Therefore, supply of a power supply potential to a part or the whole of the programmable circuit 330 can be temporarily stopped, and power supply potential can be supplied only when needed, leading to a reduction in power consumption of the semiconductor device.

The unit cells 320 are arranged in the programmable circuit 330, and the conduction state is changed between an on state and an off state in each of the unit cells 320, so that the analog value of the whole programmable circuit 330 can be changed. For example, with the use of the configuration in FIG. 1A, the following three analog values can be output: an analog value in the case where both of the unit cells 320 a and 320 b are on, an analog value in the case where one of the unit cells 320 a and 320 b is on, and an analog value in the case where both of the unit cells 320 a and 320 b are off. Thus, the programmable circuit 330 can have high versatility.

Note that in the programmable circuit 330 illustrated in FIG. 1A, the unit cell 320 may be replaced with a unit cell 321 illustrated in FIG. 1B. The unit cell 321 includes a first transistor 340, a second transistor 352, a third transistor 360, a fourth transistor 342, an inverter 370, and an analog element 310. The second transistor 352 is an n-channel transistor and the third transistor 360 is a p-channel transistor. The fourth transistor 342 includes a wide-bandgap semiconductor such as an oxide semiconductor and thus has a sufficiently low off-state current, as well as the first transistor 340.

In FIG. 1B, one of electrodes of the analog element 310 is electrically connected to an input signal line IN (not shown). One of source and drain electrodes of the second transistor 352 and one of source and drain electrodes of the third transistor 360 are electrically connected to the other of the electrodes of the analog element 310. The other of the source and drain electrodes of the second transistor 352 and the other of the source and drain electrodes of the third transistor 360 are electrically connected to an output signal line OUT (not shown). A gate electrode of the first transistor 340 and a gate electrode of the fourth transistor 342 are electrically connected to a word line W. One of source and drain electrodes of the first transistor 340 is electrically connected to a bit line B. One of source and drain electrodes of the fourth transistor 342 is electrically connected to the bit line B via the inverter 370. An inverted signal of the bit line B is supplied to the one of the source and drain electrodes of the fourth transistor 342. The other of the source and drain electrodes of the first transistor 340 is electrically connected to a gate electrode of the second transistor 350. The other of the source and drain electrodes of the fourth transistor 342 is electrically connected to a gate electrode of the third transistor 360.

As in the unit cell 320 illustrated in FIG. 1A, in the case where determination of whether the unit cell is turned on or off is performed with only the potential of the gate electrode of the second transistor 350, in order to keep the potential of the gate electrode of the second transistor 350, a potential higher than the maximum potential (or the minimum potential) applied to a source or drain electrode of the second transistor 350 by the threshold voltage of the second transistor 350 needs to be applied to the gate electrode. On the other hand, with the configuration illustrated in FIG. 1B, a potential can be held in both of a gate electrode of the second transistor 352 and the gate electrode of the third transistor 360; therefore, it is not necessary to increase the potential in consideration of a drop in voltage (or a rise in voltage) corresponding to the threshold voltage. Consequently, the power consumption of the programmable circuit can be reduced.

<Programmable Circuit 2>

FIG. 2A illustrates a configuration of a programmable circuit 332 in this embodiment, which is different from the configurations in FIGS. 1A and 1B. The programmable circuit 332 includes a unit cell 320 a, a unit cell 320 b, a unit cell 320 c, and a unit cell 320 d connected in a matrix between an input signal line IN and an output signal line OUT. Note that an embodiment of the present invention is not limited to this configuration, and the programmable circuit 332 may include five or more unit cells.

In FIG. 2A, a word line W_11 is shared by the unit cell 320 a and the unit cell 320 c. Similarly, a word line W_12 is shared by the unit cell 320 b and the unit cell 320 d. A bit line B_11 is shared by the unit cell 320 a and the unit cell 320 b. Similarly, a bit line B_21 is shared by the unit cell 320 c and the unit cell 320 d. With the use of a structure in FIG. 2A in which the unit cells 320 are arranged in a matrix, a word line W is provided for each row of unit cells, and a bit line B is provided for each column of unit cells, data can be written into a selected unit cell among the unit cells 320. Note that connections of the word lines W and the bit lines B are not limited to the arrangement shown in FIG. 2A.

The unit cells 320 included in the programmable circuit 332 and those included in the programmable circuit 330 illustrated in FIG. 1A have the same structure, so that the conduction state of each of the unit cells 320 can be controlled with a potential applied to the gate electrode of the second transistor 350 included in the unit cell 320. More specifically, a potential applied to the analog element 310 in the unit cell 320 via the input signal line IN is selectively output to the output signal line OUT when the second transistor 350 is on. Note that instead of the structure of the unit cell 320 in FIG. 1A, the structure of the unit cell 321 in FIG. 1B may be employed for the unit cells included in the programmable circuit 332.

In the programmable circuit 332, an analog value output via the output signal line OUT is a resultant total output of the unit cells 320 included in the programmable circuit 332, which means that the analog value of the programmable circuit 332 can be changed by control of output from the unit cells 320.

With the use of the programmable circuit 332 in FIG. 2A, the following four analog values can be output: an analog value in the case where all of the unit cells 320 a to 320 d are on, an analog value in the case where three of the unit cells 320 a to 320 d are on, an analog value in the case where two of the unit cells 320 a to 320 d are on, and an analog value in the case where all of the unit cells 320 a to 320 d are off. Thus, the programmable circuit 332 can have high versatility. Comparing the case where the programmable circuit 332 includes the unit cells 320 arranged in a matrix of m rows and n columns (m and n are individually an integer of 2 or more) with the case where (m×n) unit cells are connected in parallel as in the programmable circuit 330 illustrated in FIG. 1A, in the former case, the number of wirings can be reduced, resulting in a reduction in size and power consumption of the programmable circuit 332.

Note that in the case where the unit cells 320 are connected in a matrix, an output signal line OUT may be provided for each column of unit cells as in the programmable circuit 333 illustrated in FIG. 2B. In the configuration illustrated in FIG. 2B, in the case where, for example, the programmable circuit 333 includes the unit cells 320 arranged in a matrix of m rows and n columns and all of the unit cells 320 in the k-th column (2≦k≦n, k is an integer) are off, analog values output from the other columns in the programmable circuit 333 can be obtained. Note that the programmable circuit illustrated in FIG. 2B can have the same configuration as that of the programmable circuit 332, except that the programmable circuit illustrated in FIG. 2B includes a plurality of output signal lines OUT (an one output signal line is provided for each column of unit cells 320).

As described above, in each of the programmable circuits in this embodiment, the conduction state of the unit cell 320 is controlled by changing the conduction state of the second transistor 350 (by turning on or off the second transistor 350), a potential is applied to the gate electrode of the second transistor 350 via the first transistor 340 including a wide-bandgap semiconductor (e.g., oxide semiconductor) and thus having a sufficiently low off-state current. With such a structure, data determining the conduction state of the unit cell 320 can be held for a long period while supply of a power supply potential is stopped; therefore, the power consumption of the programmable circuit can be reduced. In addition, since data determining the conduction state of the unit cell 320 is held, the data does not need to be rewritten when the power supply potential is supplied again; therefore, the start time of the programmable circuit after power on can be shortened as compared to the case of using volatile memory.

Further, unlike the case where a floating gate is used in a memory unit and configuration data is written with electron injection, high voltage for injecting charge is not necessary because the unit cell 320 included in the programmable circuit in this embodiment is controlled by changing the conduction state of the first transistor 340 containing a wide bandgap semiconductor such as an oxide semiconductor. Moreover, a gate insulating layer is not deteriorated by tunneling current, which is generated when electrons are injected into a floating gate; therefore, the data rewrite cycles can be increased.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, an application example of the programmable circuit 330 in Embodiment 1 will be described. Needless to say, an analog element described in this embodiment can be applied to the programmable circuit 332. Further, an analog element other than that described in this embodiment can be applied to the programmable circuit according to one embodiment of the present invention.

Application Example 1

FIG. 3A illustrates a programmable circuit 334 in which a capacitor 312 is used as the analog element 310 in the programmable circuit 330 illustrated in FIG. 1A.

The programmable circuit 334 includes a unit cell 322 a and a unit cell 322 b connected in parallel between an input signal line IN and an output signal line OUT. The unit cells 322 (unit cell 322 a and unit cell 322 b) each include the capacitor 312 which corresponds to the analog element 310 in the unit cell 320 in FIG. 1A. One of electrodes of the capacitor 312 is electrically connected to the input signal line IN, and the other of the electrodes of the capacitor 312 is electrically connected to one of source and drain electrodes of a second transistor 350. The structure of each of the unit cells 322 is the same as that of the unit cell 320 expect for the capacitor 312.

The capacitance value output from the programmable circuit 334 illustrated in FIG. 3A can be changed by controlling the potential of the gate electrode of the second transistor included in the unit cell 322, whereby optimization of the circuit can be performed by adjusting the capacitance value. Note that as the number of unit cells 322 included in the programmable circuit 334 is larger, more varied capacitance values can be output, which is preferable.

The programmable circuit 334 can be provided as a component of an antenna circuit 400 as illustrated in FIG. 3B, for example. In the antenna circuit 400 illustrated in FIG. 3B, a plurality of capacitors 312 included in the programmable circuit 334 each serves as a resonant capacitor. In general, the value of a resonant capacitor provided in an antenna circuit is set by processing a wiring by laser cutting or the like, and the resonance frequency is adjusted; however, owing to variations in manufacturing steps, the actual resonance frequency differs from the desired resonance frequency in some cases. In addition, since the resonant capacitance is fixed, the antenna circuit has low versatility. On the other hand, the antenna circuit 400 in this embodiment includes the programmable circuit 334, so that the value of the resonant capacitor can be changed by changing the conduction state of the second transistor 350 included in the unit cell 322; thus, the resonance frequency of the antenna circuit 400 can be adjusted with a high degree of accuracy. Furthermore, the versatility of the antenna circuit 400 can be enhanced.

In addition, the capacitor 312 included in the programmable circuit 334 preferably contains a semiconductor material similar to that used for the first transistor 340 (e.g., an oxide semiconductor), in which case the process of manufacturing the programmable circuit 334 can be simplified and the manufacturing time can be shortened. Thus, the costs for manufacturing the semiconductor device can be reduced. Further, semiconductor materials contained in the first transistor 340, the second transistor 350, and the capacitor 312 may be the same.

Application Example 2

FIG. 4A illustrates a programmable circuit 336 in which a resistor 314 is used as the analog element 310 in the programmable circuit 330 illustrated in FIG. 1A.

The programmable circuit 336 includes a unit cell 324 a and a unit cell 324 b connected in parallel between an input signal line IN and an output signal line OUT. The unit cells 324 (unit cell 324 a and unit cell 324 b) each include the resistor 314 which corresponds to the analog element 310 in the unit cell 320 in FIG. 1A. One of electrodes of the resistor 314 is electrically connected to the input signal line IN, and the other of the electrodes of the resistor 314 is electrically connected to one of source and drain electrodes of a second transistor 350. The structure of each of the unit cells 324 is the same as that of the unit cell 320 expect for the resistor 314.

The resistance value output from the programmable circuit 336 illustrated in FIG. 4A can be changed by controlling the potential of the gate electrode of the second transistor included in the unit cell 324, whereby optimization of the circuit can be performed by adjusting the resistance value. Note that as the number of unit cells 324 included in the programmable circuit 336 is larger, the resistance value capable of being output is diversified, which is preferable.

The programmable circuit 336 can be provided as a component of a resistor divider circuit 402 as illustrated in FIG. 4B, for example. With the use of the programmable circuit 336 as a part of the resistor divider circuit 402, the degree of accuracy of the resistor divider circuit 402 can be increased. Furthermore, output of the resistor divider circuit 402 can be easily changed, which can enhance the versatility of the resistor divider circuit 402.

In addition, the resistor 314 included in the programmable circuit 336 preferably contains a semiconductor material similar to that used for the first transistor 340 (e.g., an oxide semiconductor), in which case the process of manufacturing the programmable circuit 336 can be simplified and the manufacturing time can be shortened. Thus, the costs for manufacturing the semiconductor device can be reduced. Further, semiconductor materials contained in the first transistor 340, the second transistor 350, and the resistor 314 may be the same.

Application Example 3

FIG. 5 illustrates a programmable circuit 338 in which a photoelectric conversion element 316 is used as the analog element 310 in the programmable circuit 330 illustrated in FIG. 1A.

The programmable circuit 338 includes a unit cell 326 a and a unit cell 326 b connected in parallel between an input signal line IN and an output signal line OUT. The unit cells 326 (unit cell 326 a and unit cell 326 b) each include the photoelectric conversion element 316 which corresponds to the analog element 310 in the unit cell 320 in FIG. 1A. One of electrodes of the photoelectric conversion element 316 is electrically connected to the input signal line IN, and the other of the electrodes of the photoelectric conversion element 316 is electrically connected to one of source and drain electrodes of a second transistor 350. The structure of each of the unit cells 326 is the same as that of the unit cell 320 expect for the photoelectric conversion element 316.

The output from the programmable circuit 338 illustrated in FIG. 5 can be changed by controlling the potential of the gate electrode of the second transistor included in the unit cell 326.

The photoelectric conversion efficiency of a photoelectric conversion element is defined as the rate at which energy of light with which the photoelectric conversion element is irradiated is converted into electric energy. In a circuit including a plurality of photoelectric conversion elements, low irradiation energy of light in a part of the circuit causes the photoelectric conversion efficiency of the whole circuit to decrease. However, in the programmable circuit 338 in this embodiment, since the conduction state can be controlled in each of the unit cells 326, by turning off the unit cells 326 provided in a part of the circuit with low irradiation energy of light, the programmable circuit 338 can operate without decreasing the photoelectric conversion efficiency of the whole programmable circuit 338.

In addition, the photoelectric conversion element 316 included in the programmable circuit 338 preferably contains a semiconductor material similar to that used for the first transistor 340 (e.g., an oxide semiconductor), in which case the process of manufacturing the programmable circuit 338 can be simplified and the manufacturing time can be shortened. Thus, the costs for manufacturing the semiconductor device can be reduced. Further, semiconductor materials contained in the first transistor 340, the second transistor 350, and the photoelectric conversion element 316 may be the same.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, a method for manufacturing transistors included in the programmable circuit in Embodiment 1 or Embodiment 2 will be described with reference to FIGS. 6A to 6D, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9A and 9B. In this embodiment, a method for manufacturing a transistor 110 including an oxide semiconductor (hereinafter also referred to as an oxide semiconductor transistor) and an n-channel transistor 112 is described as a specific example. Note that the oxide semiconductor transistor 110 corresponds to the first transistor 340 in FIG. 1A, and the n-channel transistor 112 corresponds to the second transistor 350 in FIG. 1A. In FIGS. 6A to 6D, FIGS. 7A and 7B, FIGS. 8A to 8C, and FIGS. 9A and 9B, the cross section along A-B is a cross-sectional view of a region where the oxide semiconductor transistor 110 and the n-channel transistor 112 are formed, and the cross section along C-D is a cross-sectional view of a node where one of a source electrode and a drain electrode of the oxide semiconductor transistor 110 and a gate electrode of the n-channel transistor 112 are connected to each other.

First, as illustrated in FIG. 6A, element isolation regions 203 are formed in a p-type semiconductor substrate 201.

Examples of the p-type semiconductor substrate 201 are a single crystal silicon substrate (a silicon wafer) having p-type conductivity and a compound semiconductor substrate (e.g., a SiC substrate, a sapphire substrate, and a GaN substrate).

Instead of the p-type semiconductor substrate 201, the following SOI (silicon on insulator) substrate may be used: an SIMOX (separation by implanted oxygen) substrate which is formed by implanting oxygen ions into a minor-polished wafer, and by subsequently performing high-temperature heating to form an oxide layer at a certain depth from the surface and to eliminate defects generated in a surface layer; an SOI substrate formed by a Smart-Cut method in which a semiconductor substrate is cleaved by thermal treatment by utilizing growth of minute voids, which are formed by implantation of hydrogen ions, or an SOI substrate formed by an ELTRAN (epitaxial layer transfer: a registered trademark of Canon Inc.) method or the like.

The element isolation regions 203 are formed by a LOCOS (local oxidation of silicon) method, an STI (shallow trench isolation) method, or the like.

When a p-channel transistor is formed over the same substrate as the oxide semiconductor transistor 110 and the n-channel transistor 112, for example, when the third transistor 360 and the inverter 370 in FIG. 1B are formed over the same substrate, an n-well region may be formed in part of the p-type semiconductor substrate 201. The n-well region is formed by addition of an impurity element imparting n-type conductivity to the substrate, such as phosphorus or arsenic.

Although the p-type semiconductor substrate is used here, a p-channel transistor may be formed using an n-type semiconductor substrate. In that case, a p-well region to which an impurity element imparting p-type conductivity, such as boron, is added may be formed in the n-type semiconductor substrate and an n-channel transistor may be formed over the same substrate.

Next, as illustrated in FIG. 6B, a gate insulating film 207 and a gate electrode 209 are formed over the semiconductor substrate 201.

Heat treatment is performed to oxidize a surface of the semiconductor substrate 201, thereby forming a silicon oxide film. Alternatively, a stack of a silicon oxide film and a silicon film containing oxygen and nitrogen (silicon oxynitride film) may be formed in such a manner that the silicon oxide film is formed by thermal oxidation and then nitridation treatment is performed to nitride a surface of the silicon oxide film. Next, part of the silicon oxide film or part of the silicon oxynitride film is selectively etched, thereby forming the gate insulating film 207. Alternatively, the gate insulating film 207 is formed in the following manner: a film with a thickness of 5 nm to 50 nm is formed by CVD, sputtering, or the like using silicon oxide, silicon oxynitride, or a high dielectric constant material (high-k material), for example, metal oxide such as tantalum oxide, hafnium oxide, zirconium oxide, aluminum oxide, or titanium oxide, rare earth oxide such as lanthanum oxide, or hafnium silicate and then part of the film is selectively etched.

The gate electrode 209 is preferably formed using a metal such as tantalum, tungsten, titanium, molybdenum, chromium, or niobium or an alloy material or a compound material containing any of the metals as its main component. Further, polycrystalline silicon to which an impurity element such as phosphorus is added can be used. Alternatively, the gate electrode 209 may be a stack of a metal nitride film and a film of any of the above metals. As metal nitride, tungsten nitride, molybdenum nitride, or titanium nitride can be used. The provision of the metal nitride film can increase the adhesiveness of the metal film and thus can prevent the metal film from being separated from the gate insulating film 207.

The gate electrode 209 is formed in such a manner that a conductive film is formed by sputtering, CVD, or the like and then part of the conductive film is selectively etched.

Here, a silicon oxide film is formed by performing heat treatment to oxidize a surface of the semiconductor substrate 201, a conductive film in which a tantalum nitride film and a tungsten film are stacked is formed over the silicon oxide film by sputtering, and then, part of the silicon oxide film and part of the conductive film are selectively etched, thereby forming the gate insulating film 207 and the gate electrode 209.

Note that in terms of higher integration, it is preferable that a sidewall insulating layer be not provided on a side surface of the gate electrode 209. On the other hand, a sidewall insulating layer can be provided on a side surface of the gate electrode 209 if a priority is given to characteristics of a transistor.

Next, as illustrated in FIG. 6C, an impurity element imparting n-type conductivity is added to the semiconductor substrate 201 to form n-type impurity regions 211 a and 211 b. In the case where an n-well region is formed in the semiconductor substrate 201, an impurity element imparting p-type conductivity is added to the n-well region to form p-type impurity regions. The concentration of the impurity element imparting n-type conductivity in the n-type impurity regions 211 a and 211 b and the concentration of the impurity element imparting p-type conductivity in the p-type impurity regions range from 1×10¹⁹/cm³ to 1×10²¹/cm³. The impurity element imparting n-type conductivity and the impurity element imparting p-type conductivity are added to the semiconductor substrate 201 and the n-well region, respectively, by ion doping, ion implantation, or the like as appropriate.

In the case where a sidewall insulating layer is provided on a side surface of the gate electrode 209, an impurity region having an impurity concentration different from those of the n-type impurity regions 211 a and 211 b and the p-type impurity regions can be formed in a region overlapping with the sidewall insulating layer.

Next, as illustrated in FIG. 6D, an insulating film 215 and an insulating film 217 are formed over the semiconductor substrate 201, the element isolation regions 203, the gate insulating film 207, and the gate electrode 209 by sputtering, CVD, or the like.

The insulating films 215 and 217 are each formed with a single layer or a stack of layers using one or more of silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, and the like. When the insulating film 215 is formed by CVD, a hydrogen content of the insulating film 215 is increased. When such an insulating film 215 is used and heat treatment is performed, the semiconductor substrate is hydrogenated and dangling bonds are terminated with hydrogen, so that defects in the semiconductor substrate can be reduced.

The planarity of the insulating film 217 can be high when the insulating film 217 is formed using an inorganic material such as boron phosphorus silicon glass (BPSG) or an organic material such as polyimide or acrylic.

After the insulating film 215 or the insulating film 217 is formed, heat treatment is performed in order to activate the impurity elements added to the n-type impurity regions 211 a and 211 b and the p-type impurity regions.

Through the above-described steps, the n-channel transistor 112 can be fabricated as illustrated in FIG. 6D.

Next, parts of the insulating films 215 and 217 are selectively etched to form openings. Then, a contact plug 219 a and a contact plug 219 b are formed in the openings. Typically, a conductive film is formed by sputtering, CVD, or the like and then subjected to planarization treatment such as chemical mechanical polishing (CMP) or etching so that an unnecessary portion of a surface of the conductive film is removed, thereby forming the contact plugs.

The conductive film serving as the contact plugs 219 a and 219 b is formed by depositing tungsten silicide in the openings by CVD using a WF₆ gas and a SiH₄ gas.

Next, an insulating film is formed over the insulating film 217 and the contact plugs 219 a and 219 b by sputtering, CVD, or the like, and after that, part of the insulating film is selectively etched to form an insulating film 221 comprising openings. Subsequently, a conductive film is formed by sputtering, CVD, or the like and then subjected to planarization treatment such as CMP or etching so that an unnecessary portion of a surface of the conductive film is removed, thereby forming a wiring 223 a and a wiring 223 b (see FIG. 7A).

Here, the wirings 223 a and 223 b function as a source electrode and a drain electrode of the transistor 112. One of the wirings 223 a and 223 b is electrically connected to one electrode of the analog element 310 illustrated in FIG. 1A, and the other of the wirings is electrically connected to the output signal line OUT illustrated in FIG. 1A.

The insulating film 221 can be formed using a material similar to that used for the insulating film 215.

The wirings 223 a and 223 b are formed with a single layer or a stack of layers containing a metal such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, or tungsten or an alloy containing any of these metals as a main component. For example, the wirings 223 a and 223 b have a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a tungsten film, a two-layer structure in which a copper film is formed over a copper-magnesium-aluminum alloy film, or a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used.

With the use of the planarized insulating film 221 and wirings 223 a and 223 b, variations in electric characteristics of oxide semiconductor transistors to be fabricated later can be reduced, and the oxide semiconductor transistors can be fabricated at high yield.

Then, hydrogen contained in the insulating film 221 and the wirings 223 a and 223 b is preferably eliminated by heat treatment or plasma treatment. Consequently, in a heat treatment performed after formation of an insulating film and oxide semiconductor film on the insulating film 221 and the wirings 223 a and 223 b, diffusion of hydrogen into the insulating film and the oxide semiconductor film can be prevented. The heat treatment is performed at a temperature higher than or equal to 100° C. and lower than the strain point of the substrate in an inert atmosphere, a reduced-pressure atmosphere, or a dry air atmosphere. Further, for the plasma treatment, a rare gas, oxygen, nitrogen, or nitrogen oxide (e.g., nitrous oxide, nitrogen monoxide, or nitrogen dioxide) is used.

Next, an insulating film 225 is formed over the insulating film 221 and the wirings 223 a and 223 b by sputtering, CVD, or the like. The insulating film 225 is formed with a single layer or a stack of layers containing silicon oxide, silicon oxynitride, silicon nitride oxide, gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, and/or aluminum oxynitride. Moreover, the insulating film 225 is preferably formed using an oxide insulating film from which oxygen is released by heating. The oxide insulating film from which oxygen is released by heating is an oxide insulating film that contains oxygen at a proportion exceeding the stoichiometric proportion. Since oxygen is released from such an oxide insulating film by heating, oxygen can be diffused into the oxide semiconductor film by heat treatment performed later.

The insulating film 225 is preferably planarized by CMP treatment or the like. The average surface roughness (R_(a)) of a surface of the insulating film 225 is 1 nm or less, preferably 0.3 nm or less, further preferably 0.1 nm or less.

In this specification and the like, the average surface roughness (R_(a)) is obtained by three-dimension expansion of center line average surface roughness (R_(a)) which is defined by JISB0601:2001 (ISO 4287:1997) so that R_(a) can be applied to a measurement surface, and is an average value of the absolute values of deviations from a reference surface to a specific surface.

When the measurement surface which is a surface represented by measurement data is expressed by Z=F(X,Y), the average surface roughness (R_(a)) is an average value of the absolute values of deviations from the reference surface to the specific surface and is expressed by Formula 1.

$\begin{matrix} {R_{a} = {\frac{1}{S_{0}}{\int_{Y_{1}}^{Y_{2}}{\int_{X_{1}}^{X_{2}}{{{{F\left( {X,Y} \right)} - Z_{0}}}\ {\mathbb{d}X}\ {\mathbb{d}Y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

Here, the specific surface is a surface which is a target of roughness measurement, and is a rectangular region surrounded by four points represented by the coordinates (X₁,Y₁), (X₁,Y₂), (X₂,Y₁), and (X₂,Y₂). Moreover, S₀ represents the area of the specific surface when the specific surface is flat ideally. The reference surface refers to a surface parallel to an X-Y surface at the average height of the specific surface. That is, when the average value of the height of the specific surface is denoted by Z₀, the height of the reference surface is also denoted by Z₀. The average surface roughness (R_(a)) can be measured using an atomic force microscope (AFM).

The CMP treatment may be performed once or plural times. When the CMP treatment is performed plural times, first polishing is preferably performed with a high polishing rate followed by final polishing with a low polishing rate. By performing polishing steps with different polishing rates in combination, the flatness of the surface of the insulating film 225 can be further increased.

Alternatively, the insulating film 225 can be planarized by plasma treatment. The plasma treatment is performed in such a manner that an inert gas, for example, a rare gas such as an argon gas is introduced into a vacuum chamber and an electric field is applied so that a surface to be processed serves as a cathode. The plasma treatment has the same principle as a plasma dry etching method, except that an inert gas is used in the plasma treatment. In other words, in the plasma treatment, the surface to be processed is irradiated with ions of an inert gas and minute unevenness of the surface is reduced by a sputtering effect. Accordingly, the plasma treatment can be called reverse sputtering treatment.

When the plasma treatment is performed, electrons and argon cations are present in plasma and the argon cations are accelerated in the cathode direction. The surface to be processed is sputtered by the accelerated argon cations. At that time, a projected portion of the surface is preferentially sputtered. Particles ejected from the surface attach to another place of the surface. At that time, the particles ejected from the surface preferentially attach to a recessed portion of the surface. By thus reducing the projected portion and filling the recessed portion, the planarity of the surface is increased. Note that the use of the plasma treatment and the CMP treatment in combination can further increase the planarity of the insulating film 225.

Note that through the plasma treatment, it is possible to remove impurities, such as oxygen, moisture, and an organic compound, attached onto the surface of the insulating film 225 by a sputtering effect.

Note that before the deposition of the oxide semiconductor film, it is preferable that a deposition chamber be heated and evacuated to remove impurities, such as hydrogen, water, a hydroxyl group, and hydride, in the deposition chamber. It is particularly important to remove the impurities absorbed on an inner wall of the deposition chamber. Here, heat treatment may be performed at 100° C. to 450° C., for example. The deposition chamber is preferably evacuated with a rough vacuum pump such as a dry pump and a high vacuum pump such as a sputter ion pump, a turbo molecular pump, or a cryopump in appropriate combination. The turbo molecular pump has an outstanding capability of evacuating large-sized molecules, whereas it has a low capability of evacuating hydrogen or water. Hence, it is effective to use a combination of the turbo molecular pump and a cryopump having a high capability of evacuating water or a sputter ion pump having a high capability of evacuating hydrogen. At this time, when impurities are removed while an inert gas is introduced, the rate of elimination of water or the like, which is difficult to eliminate only by evacuation, can be further increased. By removing impurities in the deposition chamber by such treatment before the deposition of the oxide semiconductor film, mixture of hydrogen, water, a hydroxyl group, hydride, and the like into the oxide semiconductor can be reduced.

In addition, before the oxide semiconductor film is formed with a sputtering apparatus, a dummy substrate may be put into the sputtering apparatus, and an oxide semiconductor film may be formed over the dummy substrate to remove hydrogen and moisture attached to the target surface or a deposition shield.

Next, an oxide semiconductor film 227 is formed over the insulating film 225 by sputtering, coating, printing, evaporation, PCVD, PLD, ALD, MBE, or the like (see FIG. 7B). Here, the oxide semiconductor film 227 is formed to a thickness of 1 nm to 50 nm, preferably 3 nm to 20 nm by sputtering. With the oxide semiconductor film 227 having a thickness in the above range, a short-channel effect which might occur due to miniaturization of the transistor can be suppressed.

An oxide semiconductor used for the oxide semiconductor film 227 preferably contains at least indium (In) or zinc (Zn). In particular, the oxide semiconductor preferably contains In and Zn. As a stabilizer for reducing variations in electric characteristics of transistors using the oxide semiconductor, the oxide semiconductor preferably contains gallium (Ga), tin (Sn), hafnium (Hf), and/or aluminum (Al) in addition to In and Zn.

As another stabilizer, the oxide semiconductor may contain one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

As the oxide semiconductor, any of the following oxides can be used, for example: indium oxide; tin oxide; zinc oxide; two-component metal oxide such as In—Zn-based oxide, Sn—Zn-based oxide, Al—Zn-based oxide, Zn—Mg-based oxide, Sn—Mg-based oxide, In—Mg-based oxide, and In—Ga-based oxide; three-component metal oxide such as In—Ga—Zn-based oxide (also referred to as IGZO), In—Al—Zn-based oxide, In—Sn—Zn-based oxide, Sn—Ga—Zn-based oxide, Al—Ga—Zn-based oxide, Sn—Al—Zn-based oxide, In—Hf—Zn-based oxide, In—La—Zn-based oxide, In—Ce—Zn-based oxide, In—Pr—Zn-based oxide, In—Nd—Zn-based oxide, In—Sm—Zn-based oxide, In—Eu—Zn-based oxide, In—Gd—Zn-based oxide, In—Tb—Zn-based oxide, In—Dy—Zn-based oxide, In—Ho—Zn-based oxide, In—Er—Zn-based oxide, In—Tm—Zn-based oxide, In—Yb—Zn-based oxide, and In—Lu—Zn-based oxide; and four-component metal oxide such as In—Sn—Ga—Zn-based oxide, In—Hf—Ga—Zn-based oxide, In—Al—Ga—Zn-based oxide, In—Sn—Al—Zn-based oxide, In—Sn—Hf—Zn-based oxide, and In—Hf—Al—Zn-based oxide. Moreover, silicon oxide may be included in the above oxide semiconductor. Here, for example, an In—Ga—Zn-based oxide means an oxide containing indium (In), gallium (Ga), and zinc (Zn) as its main components and there is no particular limitation on the composition ratio. The In—Ga—Zn-based oxide may contain a metal element other than In, Ga, and Zn. Here, the amount of oxygen in the above oxide semiconductor preferably exceeds the stoichiometric proportion of oxygen. When the amount of oxygen exceeds the stoichiometric proportion, generation of carriers which results from oxygen vacancies in the oxide semiconductor film can be suppressed.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 and m is not an integer) may be used as the oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Alternatively, as the oxide semiconductor, a material expressed by In₃SnO₅(ZnO)_(n) (n>0 and n is an integer) may be used.

The concentration of alkali metals or alkaline earth metals in the oxide semiconductor film 227 is preferably lower than or equal to 1×10¹⁸ atoms/cm³, further preferably lower than or equal to 2×10¹⁶ atoms/cm³. This is because carriers might be generated when an alkali metal or an alkaline earth metal is bonded to an oxide semiconductor, which causes an increase in the off-state current of the transistor.

The oxide semiconductor film 227 may contain nitrogen at a concentration of 5×10¹⁸ atoms/cm³ or lower.

The oxide semiconductor that can be used for the oxide semiconductor film 227 is a wide bandgap semiconductor, which has a wider bandgap and a lower intrinsic carrier density than a silicon semiconductor. The off-state current of the transistor can be reduced by using an oxide semiconductor with a wide energy gap.

The oxide semiconductor film 227 may have a single crystal structure or a non-single-crystal structure. In the latter case, the oxide semiconductor film 227 may have an amorphous structure or a polycrystalline structure. Further, the oxide semiconductor film 227 may have an amorphous structure including a portion having crystallinity or a non-amorphous structure.

An oxide semiconductor in an amorphous state can have a flat surface with relative ease; thus, when a transistor is fabricated using the oxide semiconductor, interface scattering can be reduced and relatively high mobility can be obtained with relative ease.

In an oxide semiconductor having crystallinity, defects in the bulk can be further reduced, and when the surface flatness is increased, mobility higher than that of an oxide semiconductor in an amorphous state can be obtained. In order to increase the surface flatness, the oxide semiconductor is preferably formed on a flat surface. As described above, the oxide semiconductor film 227 is preferably formed over a surface of the insulating film 225 with the average surface roughness (R_(a)) of 1 nm or less, preferably 0.3 nm or less, further preferably 0.1 nm or less.

Here, the oxide semiconductor film 227 is formed by sputtering.

As a sputtering target, any of the following oxides can be used, for example: indium oxide; tin oxide; zinc oxide; two-component metal oxide such as In—Zn-based oxide, Sn—Zn-based oxide, Al—Zn-based oxide, Zn—Mg-based oxide, Sn—Mg-based oxide, In—Mg-based oxide, and In—Ga-based oxide; three-component metal oxide such as In—Ga—Zn-based oxide, In—Al—Zn-based oxide, In—Sn—Zn-based oxide, Sn—Ga—Zn-based oxide, Al—Ga—Zn-based oxide, Sn—Al—Zn-based oxide, In—Hf—Zn-based oxide, In—La—Zn-based oxide, In—Ce—Zn-based oxide, In—Pr—Zn-based oxide, In—Nd—Zn-based oxide, In—Sm—Zn-based oxide, In—Eu—Zn-based oxide, In—Gd—Zn-based oxide, In—Tb—Zn-based oxide, In—Dy—Zn-based oxide, In—Ho—Zn-based oxide, In—Er—Zn-based oxide, In—Tm—Zn-based oxide, In—Yb—Zn-based oxide, and In—Lu—Zn-based oxide; and four-component metal oxide such as In—Sn—Ga—Zn-based oxide, In—Hf—Ga—Zn-based oxide, In—Al—Ga—Zn-based oxide, In—Sn—Al—Zn-based oxide, In—Sn—Hf—Zn-based oxide, and In—Hf—Al—Zn-based oxide.

When an In—Ga—Zn—O-based material is used as the oxide semiconductor, a ratio of atoms of the metal elements of the target is In:Ga:Zn=1:1:1, 4:2:3, 3:1:2, 1:1:2, 2:1:3, or 3:1:4 in an atomic ratio, for example. An In—Ga—Zn-based oxide with such an atomic ratio or an oxide whose composition is in the neighborhood of the above compositions can be used as the target. With the target with the above composition ratio, a polycrystalline film or a CAAC-OS film described later is likely to be formed.

When an In—Sn—Zn—O-based material is used as the oxide semiconductor, a ratio of atoms of the metal elements of the target is In:Sn:Zn=1:1:1, 2:1:3, 1:2:2, or 20:45:35, for example, in an atomic ratio is used. An In—Sn—Zn-based oxide with such an atomic ratio or an oxide whose composition is in the neighborhood of the above compositions can be used as the target. With the target with the above composition ratio, a polycrystalline film or a CAAC-OS film described later is likely to be formed.

When an In—Zn—O-based material is used as the oxide semiconductor, a ratio of atoms of the metal elements of the target is In:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for forming an In—Zn-based oxide semiconductor that has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied. An In—Zn-based oxide with such an atomic ratio or an oxide whose composition is in the neighborhood of the above compositions can be used as the target.

However, without limitation to the materials given above, a material with an appropriate composition may be used depending on needed semiconductor characteristics (e.g., mobility, threshold voltage, and variation). In order to obtain the needed semiconductor characteristics, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio between a metal element and oxygen, the interatomic distance, the density, and the like be set to appropriate values.

For example, high mobility can be obtained relatively easily in the case of using an In—Sn—Zn-based oxide. However, mobility can be increased by reducing the defect density in a bulk also in the case of using an In—Ga—Zn-based oxide.

Note that for example, the expression “the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in the neighborhood of the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b, and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r may be 0.05, for example. The same applies to other oxides.

As a sputtering gas, a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen is preferably higher than that of the rare gas. Further, in order to prevent hydrogen, water, a hydroxyl group, a hydride, and the like from entering the oxide semiconductor film, the sputtering gas is preferably an atmosphere of a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, and a hydride are sufficiently removed.

For sputtering, an RF power supply device, an AC power supply device, a DC power supply device, or the like can be used as appropriate as a power supply device for generating plasma.

Note that the leakage rate of a treatment chamber in which the oxide semiconductor film is formed is preferably lower than or equal to 1×10⁻¹⁰ Pa·m³/s, whereby entry of impurities into the film to be formed by sputtering can be decreased. As described above, in the process of forming the oxide semiconductor film and preferably in the process of forming the oxide insulating film, entry of impurities is suppressed as much as possible through control of the pressure of the treatment chamber, the leakage rate of the treatment chamber, and the like, whereby entry of impurities such as hydrogen into the oxide semiconductor film can be reduced. In addition, diffusion of impurities such as hydrogen from the oxide insulating film to the oxide semiconductor film can be reduced.

An oxide semiconductor film can be single crystal, polycrystalline (also referred to as polycrystal), or amorphous, for example. As the oxide semiconductor film, a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film may be used.

The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film with a crystal-amorphous mixed phase structure where crystalline parts and amorphous parts are included in an amorphous phase. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous part and a crystal part in the CAAC-OS film is not clear. Further, a grain boundary in the CAAC-OS film is not found with the TEM. Thus, a reduction in electron mobility due to the grain boundary is suppressed in the CAAC-OS film.

In each of the crystal parts included in the CAAC-OS film, the c-axis is aligned in a direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, when crystal growth occurs from a surface side of the oxide semiconductor film in the process of forming the CAAC-OS film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is sometimes higher than that in the vicinity of the surface where the oxide semiconductor film is formed. Further, when an impurity is added to the CAAC-OS film, the crystal part in a region to which the impurity is added becomes amorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that the direction of c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by deposition or by performing treatment for crystallization such as heat treatment after deposition.

With the use of the CAAC-OS film, the change in electric characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Specifically, as described above, it is preferable that the average surface roughness (R_(a)) of the surface of the insulating film 225 be 1 nm or less, preferably 0.3 nm or less, further preferably 0.1 nm or less and a CAAC-OS film be formed over the surface of the insulating film 225. Thus, the crystallinity of the CAAC-OS film can be increased, and the mobility of the transistor including the CAAC-OS film can be increased.

Examples of a crystal structure of the CAAC-OS film will be described in detail with reference to FIGS. 12A to 12E, FIGS. 13A to 13C, FIGS. 14A to 14C, and FIGS. 15A and 15B. In FIGS. 12A to 12E, FIGS. 13A to 13C, FIGS. 14A to 14C, and FIGS. 15A and 15B, the vertical direction corresponds to the c-axis direction and a plane perpendicular to the c-axis direction corresponds to the a-b plane, unless otherwise specified. An “upper half” and a “lower half” refer to an upper half above the a-b plane and a lower half below the a-b plane (an upper half and a lower half with respect to the a-b plane). Furthermore, in FIGS. 12A to 12E, O surrounded by a circle represents tetracoordinate O and O surrounded by a double circle represents tricoordinate O.

FIG. 12A illustrates a structure including one hexacoordinate In atom and six tetracoordinate oxygen (hereinafter referred to as tetracoordinate O) atoms proximate to the In atom. Here, a structure including one metal atom and oxygen atoms proximate thereto is referred to as a small group. The structure in FIG. 12A is actually an octahedral structure, but is illustrated as a planar structure for simplicity. Note that three tetracoordinate O atoms exist in each of the upper half and the lower half in FIG. 12A. The electric charge of the small group in FIG. 12A is 0.

FIG. 12B illustrates a structure including one pentacoordinate Ga atom, three tricoordinate oxygen (hereinafter referred to as tricoordinate O) atoms proximate to the Ga atom, and two tetracoordinate O atoms proximate to the Ga atom. All the tricoordinate O atoms exist on the a-b plane. One tetracoordinate O atom exists in each of the upper half and the lower half in FIG. 12B. An In atom can also have the structure illustrated in FIG. 12B because an In atom can have five ligands. The electric charge of the small group in FIG. 12B is 0.

FIG. 12C illustrates a structure including one tetracoordinate Zn atom and four tetracoordinate O atoms proximate to the Zn atom. In FIG. 12C, one tetracoordinate O atom exists in the upper half and three tetracoordinate O atoms exist in the lower half. The electric charge of the small group in FIG. 12C is 0.

FIG. 12D illustrates a structure including one hexacoordinate Sn atom and six tetracoordinate O atoms proximate to the Sn atom. In FIG. 12D, three tetracoordinate O atoms exist in each of the upper half and the lower half. The electric charge of the small group in FIG. 12D is +1.

FIG. 12E illustrates a small group including two Zn atoms. In FIG. 12E, one tetracoordinate O atom exists in each of the upper half and the lower half The electric charge of the small group in FIG. 12E is −1.

Here, a plurality of small groups form a medium group, and a plurality of medium groups form a large group.

Now, a rule of bonding between the small groups will be described. The three O atoms in the upper half with respect to the In atom each have three proximate In atoms in the downward direction, and the three O atoms in the lower half each have three proximate In atoms in the upward direction. The one O atom in the upper half with respect to the Ga atom has one proximate Ga atom in the downward direction, and the one O atom in the lower half has one proximate Ga atom in the upward direction. The one O atom in the upper half with respect to the Zn atom has one proximate Zn atom in the downward direction, and the three O atoms in the lower half each have three proximate Zn atoms in the upward direction. In this manner, the number of tetracoordinate O atoms above a metal atom is equal to the number of metal atoms proximate to and below the tetracoordinate O atoms; similarly, the number of tetracoordinate O atoms below a metal atom is equal to the number of metal atoms proximate to and above the tetracoordinate O atoms. Since the coordination number of the tetracoordinate O atom is 4, the sum of the number of metal atoms proximate to and below the O atom and the number of metal atoms proximate to and above the O atom is 4. Accordingly, when the sum of the number of tetracoordinate O atoms above a metal atom and the number of tetracoordinate O atoms below another metal atom is 4, the two kinds of small groups including the metal atoms can be bonded to each other. For example, in the case where the hexacoordinate metal (In or Sn) atom is bonded through three tetracoordinate O atoms in the upper half, it is bonded to the pentacoordinate metal (Ga or In) atom or the tetracoordinate metal (Zn) atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded to another metal atom through a tetracoordinate O atom in the c-axis direction. In addition, a medium group can be formed in a different manner by combining a plurality of small groups so that the total electric charge of the layered structure is 0.

FIG. 13A illustrates a model of a medium group included in a layered structure of an In—Sn—Zn—O-based material. FIG. 13B illustrates a large group including three medium groups. Note that FIG. 13C illustrates an atomic arrangement where the layered structure in FIG. 13B is observed from the c-axis direction.

In FIG. 13A, a tricoordinate O atom is omitted for simplicity, and a tetracoordinate O atom is illustrated by a circle; the number in the circle shows the number of tetracoordinate O atoms. For example, three tetracoordinate O atoms existing in each of the upper half and the lower half with respect to a Sn atom is denoted by circled 3. Similarly, in FIG. 13A, one tetracoordinate O atom existing in each of the upper half and the lower half with respect to an In atom is denoted by circled 1. FIG. 13A also illustrates a Zn atom proximate to one tetracoordinate O atom in the lower half and three tetracoordinate O atoms in the upper half, and a Zn atom proximate to one tetracoordinate O atom in the upper half and three tetracoordinate O atoms in the lower half.

In the medium group included in the layered structure of the In—Sn—Zn—O-based material in FIG. 13A, in the order starting from the top, a Sn atom proximate to three tetracoordinate O atoms in each of the upper half and the lower half is bonded to an In atom proximate to one tetracoordinate O atom in each of the upper half and the lower half, the In atom is bonded to a Zn atom proximate to three tetracoordinate O atoms in the upper half, the Zn atom is bonded to an In atom proximate to three tetracoordinate O atoms in each of the upper half and the lower half through one tetracoordinate O atom in the lower half with respect to the Zn atom, the In atom is bonded to a small group that includes two Zn atoms and is proximate to one tetracoordinate O atom in the upper half, and the small group is bonded to a Sn atom proximate to three tetracoordinate O atoms in each of the upper half and the lower half through one tetracoordinate O atom in the lower half with respect to the small group. A plurality of such medium groups are bonded to form a large group.

Here, electric charge for one bond of a tricoordinate O atom and electric charge for one bond of a tetracoordinate O atom can be assumed to be −0.667 and −0.5, respectively. For example, electric charge of a (hexacoordinate or pentacoordinate) In atom, electric charge of a (tetracoordinate) Zn atom, and electric charge of a (pentacoordinate or hexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly, electric charge in a small group including a Sn atom is +1. Therefore, electric charge of −1, which cancels +1, is needed to form a layered structure including a Sn atom. As a structure having electric charge of −1, the small group including two Zn atoms as illustrated in FIG. 12E can be given. For example, with one small group including two Zn atoms, electric charge of one small group including a Sn atom can be cancelled, so that the total electric charge of the layered structure can be 0.

Specifically, by repeating the large group illustrated in FIG. 13B, an In—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that a layered structure of the obtained In—Sn—Zn—O-based crystal can be expressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or a natural number).

The above-described rule also applies to the following oxides: four-component metal oxide such as In—Sn—Ga—Zn-based oxide; three-component metal oxide such as In—Ga—Zn-based oxide, In—Al—Zn-based oxide, Sn—Ga—Zn-based oxide, Al—Ga—Zn-based oxide, Sn—Al—Zn-based oxide, In—Hf—Zn-based oxide, In—La—Zn-based oxide, In—Ce—Zn-based oxide, In—Pr—Zn-based oxide, In—Nd—Zn-based oxide, In—Sm—Zn-based oxide, In—Eu—Zn-based oxide, In—Gd—Zn-based oxide, In—Tb—Zn-based oxide, In—Dy—Zn-based oxide, In—Ho—Zn-based oxide, In—Er—Zn-based oxide, In—Tm—Zn-based oxide, In—Yb—Zn-based oxide, and In—Lu—Zn-based oxide; two-component metal oxide such as an In—Zn-based oxide, Sn—Zn-based oxide, Al—Zn-based oxide, Zn—Mg-based oxide, Sn—Mg-based oxide, In—Mg-based oxide, and In—Ga-based oxide; and single-component metal oxide such as In-based oxide, Sn-based oxide, and Zn-based oxide.

As an example, FIG. 14A illustrates a model of a medium group included in a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of the In—Ga—Zn—O-based material in FIG. 14A, in the order starting from the top, an In atom proximate to three tetracoordinate O atoms in each of the upper half and the lower half is bonded to a Zn atom proximate to one tetracoordinate O atom in the upper half, the Zn atom is bonded to a Ga atom proximate to one tetracoordinate O atom in each of the upper half and the lower half through three tetracoordinate O atoms in the lower half with respect to the Zn atom, and the Ga atom is bonded to an In atom proximate to three tetracoordinate O atoms in each of the upper half and the lower half through one tetracoordinate O atom in the lower half with respect to the Ga atom. A plurality of such medium groups are bonded to form a large group.

FIG. 14B illustrates a large group including three medium groups. Note that FIG. 14C illustrates an atomic arrangement where the layered structure in FIG. 14B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) In atom, electric charge of a (tetracoordinate) Zn atom, and electric charge of a (pentacoordinate) Ga atom are +3, +2, +3, respectively, electric charge of a small group including any of an In atom, a Zn atom, and a Ga atom is 0. As a result, the total electric charge of a medium group having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material, a large group can be formed using not only the medium group illustrated in FIG. 14A but also a medium group in which the arrangement of the In atom, the Ga atom, and the Zn atom is different from that in FIG. 14A.

Specifically, by repeating the large group illustrated in FIG. 14B, an In—Ga—Zn—O-based crystal can be obtained. Note that a layered structure of the obtained In—Ga—Zn—O-based crystal can be expressed as a composition formula, InGaO₃(ZnO)_(n) (n is a natural number).

In the case of n=1 (InGaZnO₄), a crystal structure illustrated in FIG. 15A can be obtained, for example. Note that in the crystal structure in FIG. 15A, Ga can be replaced with In because a Ga atom and an In atom each have five ligands as described with reference to FIG. 12B.

In the case of n=2 (InGaZn₂O₅), a crystal structure illustrated in FIG. 15B can be obtained, for example. Note that in the crystal structure in FIG. 15B, Ga can be replaced with In because a Ga atom and an In atom each have five ligands as described with reference to FIG. 12B.

In order to form a CAAC-OS film as the oxide semiconductor film 227, the substrate is heated at a temperature higher than 200° C. and lower than or equal to 700° C., preferably higher than 300° C. and lower than or equal to 500° C., further preferably higher than or equal to 400° C. and lower than or equal to 450° C. during the deposition of the oxide semiconductor film 227. The oxide semiconductor film 227 is deposited while the substrate is heated in the above manner, whereby the oxide semiconductor film 227 can be a CAAC-OS film.

Alternatively, the oxide semiconductor film 227 may be formed in the following manner to be a CAAC-OS film. A first oxide semiconductor film with a small thickness, specifically in the range from the thickness of one atomic layer to 10 nm, preferably from 2 nm to 5 nm, is deposited while heating is performed in the above-described temperature range. Then, a second oxide semiconductor film with a larger thickness is deposited in conditions similar to the first oxide semiconductor film, thereby forming a stack of the first oxide semiconductor film and the second oxide semiconductor film.

Further, in order to make the oxide semiconductor film 227 have an amorphous structure, the substrate is not heated or the substrate is heated at a temperature lower than 200° C., preferably lower than 180° C. during the deposition of the oxide semiconductor film 227. The oxide semiconductor film 227 deposited in such a manner can have an amorphous structure.

In addition, after the oxide semiconductor film having an amorphous structure is formed in the above manner, heat treatment may be performed at 250° C. to 700° C., preferably 400° C. or higher, further preferably 500° C. or higher, still further preferably 550° C. or higher to crystallize at least part of the oxide semiconductor film having an amorphous structure, whereby the oxide semiconductor film 227 can be formed to be a CAAC-OS film. Note that the heat treatment can be performed in an inert gas atmosphere. As the inert gas atmosphere, it is preferable to use an atmosphere that contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, or the like. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (i.e., the impurity concentration is 1 ppm or less, preferably 0.1 ppm or less). Heat treatment for dehydration or dehydrogenation, which is described later, or the like can double as this heat treatment.

As the substrate heating temperature during the deposition is higher in the above method, the impurity concentration of the obtained oxide semiconductor film 227 is decreased. Further, the atomic arrangement in the oxide semiconductor film 227 is ordered and the density of the oxide semiconductor film 227 is increased, so that a polycrystal film or a CAAC-OS film is likely to be formed. Furthermore, when an oxygen gas atmosphere is employed for the deposition, a polycrystal film or a CAAC-OS film is easily formed because an unnecessary atom such as a rare gas is not contained in the oxide semiconductor film 227. Note that a mixed gas atmosphere including an oxygen gas and a rare gas may be used. In that case, the percentage of an oxygen gas is 30 vol % or higher, preferably 50 vol % or higher, further preferably 80 vol % or higher.

After the formation of the oxide semiconductor film 227, heat treatment may be performed on the oxide semiconductor film 227. The heat treatment can further remove substances containing hydrogen atoms in the oxide semiconductor film 227. The heat treatment is performed in an inert gas atmosphere at a temperature higher than or equal to 300° C. and lower than or equal to 700° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C. (lower than the strain point of the substrate if the substrate has one). As the inert gas atmosphere, it is preferable to use an atmosphere that contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, or the like. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (i.e., the impurity concentration is 1 ppm or less, preferably 0.1 ppm or less).

For example, after the semiconductor substrate 201 is introduced into an electric furnace including a resistance heater or the like, the heat treatment can be performed at 450° C. for one hour in a nitrogen atmosphere.

The heat treatment apparatus is not limited to the electric furnace and may be an apparatus for heating an object by thermal conduction or thermal radiation from a medium such as a heated gas. For example, a rapid thermal annealing (RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object by heat treatment, like nitrogen or a rare gas such as argon is used. When a GRTA apparatus is used as the heat treatment apparatus, the substrate may be heated in an inert gas heated to high temperature of 650° C. to 700° C. because the heat treatment time is short.

In addition, it is preferable that after the oxide semiconductor film 227 is heated through the heat treatment, a high-purity oxygen gas, a high-purity N₂O gas, or ultra-dry air (with a moisture content of 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, further preferably 10 ppb or less, in the case where measurement is performed with the use of a dew point meter of a cavity ring down laser spectroscopy (CRDS) system) may be introduced into the same furnace. It is particularly preferable that water, hydrogen, and the like be not contained in these gases. The purity of the oxygen gas or the N₂O gas introduced into the heat treatment apparatus is preferably 6N or higher, further preferably 7N or higher (i.e., the impurity concentration in the oxygen gas or the N₂O gas is preferably 1 ppm or less, further preferably 0.1 ppm or less). By the action of the oxygen gas or the N₂O gas, oxygen which is one of a main component of the oxide semiconductor and which has been reduced through the step for removing impurities by dehydration or dehydrogenation treatment can be supplied.

The above heat treatment has an effect of removing hydrogen, water, and the like and can be referred to as dehydration, dehydrogenation, or the like. The heat treatment can be performed, for example, before the oxide semiconductor film is processed into an island shape or after the gate insulating film is formed. Such heat treatment for dehydration or dehydrogenation may be conducted once or plural times.

Next, part of the oxide semiconductor film 227 is selectively etched, thereby forming an island-shaped oxide semiconductor film 229. After that, an insulating film 231 is formed over the oxide semiconductor film 229 by sputtering, CVD, or the like. Then, a gate electrode 233 is formed over the insulating film 231 (see FIG. 8A).

The insulating film 231 is formed with a single layer or a stack of layers using, for example, one or more of silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide, Ga—Zn-based metal oxide, and the like. The insulating film 231 may be an oxide insulating film from which oxygen is released by heating, such as a film applicable to the insulating film 225. By using a film from which oxygen is released by heating as the insulating film 231, oxygen vacancies that are formed in the oxide semiconductor film 229 can be reduced by a subsequent heat treatment, and deterioration in electric characteristics of the transistor can be suppressed.

The insulating film 231 may be formed using a high-k material such as hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen is added, hafnium aluminate to which nitrogen is added, hafnium oxide, or yttrium oxide, in which case gate leakage current can be decreased even if the thickness of the gate insulating film is reduced.

The thickness of the insulating film 231 is preferably from 10 nm to 300 nm, further preferably from 5 nm to 50 nm, still further preferably from 10 nm to 30 nm.

The gate electrode 233 can be formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing any of these metal elements in combination; or the like. Further, manganese and/or zirconium may be used. The gate electrode 233 may have a single-layer structure or a layered structure of two or more layers. For example, the gate electrode 233 can have a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film, or a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order. Alternatively, the gate electrode 233 may be formed using a film, an alloy film, or a nitride film that contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium.

The gate electrode 233 can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. Moreover, the gate electrode 233 can have a layered structure using the above light-transmitting conductive material and the above metal element.

The gate electrode 233 is formed by a printing method or an inkjet method. Alternatively, the gate electrode 233 is formed in such a manner that a conductive film is formed by sputtering, CVD, evaporation, or the like and then part of the conductive film is selectively etched.

As a material layer in contact with the insulating film 231, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a film of metal nitride (such as InN or ZnN) is preferably provided between the gate electrode 233 and the insulating film 231. These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher and can make the threshold voltage of the transistor positive. Accordingly, a “normally-off” switching element can be obtained. For example, in the case of using an In—Ga—Zn—O film containing nitrogen, an In—Ga—Zn—O film having a nitrogen concentration at least higher than that of the oxide semiconductor film 229, or specifically, an In—Ga—Zn—O film having a nitrogen concentration of 7 at. % or higher can be used.

After the formation of the insulating film 231, heat treatment may be performed in an inert gas atmosphere or an oxygen atmosphere. The heat treatment temperature is preferably 200° C. to 450° C., further preferably 250° C. to 350° C. With such heat treatment, variations in electric characteristics of transistors including the oxide semiconductor film 229 can be reduced. Further, in the case where the insulating film 225 or the insulating film 231, each of which is in contact with the oxide semiconductor film 229, includes oxygen, oxygen can be supplied to the oxide semiconductor film 229, and oxygen vacancies in the oxide semiconductor film 229 can be filled. As described above, the heat treatment has an effect of supplying oxygen and therefore can be called treatment for “supply of oxygen”.

Note that in this embodiment, the heat treatment for supply of oxygen is performed after the insulating film 231 is formed; however, the timing of the heat treatment for supply of oxygen is not limited thereto.

As described above, the heat treatment for dehydration or dehydrogenation and the heat treatment for supply of oxygen are performed to reduce impurities and fill oxygen vacancies in the oxide semiconductor film 229, whereby the oxide semiconductor film 229 can be highly purified so as to contain impurities other than main components of the oxide semiconductor film 229 as few as possible.

Next, a dopant may be added to the oxide semiconductor film 229 using the gate electrode 233 as a mask. In that case, a first region 235 a that is covered with the gate electrode 233 and is thus not doped with the dopant, and a pair of second regions 235 b and 235 c containing the dopant are formed as illustrated in FIG. 8B. Since the dopant is added using the gate electrode 233 as a mask, the first region 235 a to which the dopant is not added and the pair of second regions 235 b and 235 c containing the dopant can be formed in a self-aligned manner. Note that the first region 235 a which overlaps with the gate electrode 233 functions as a channel region. The second regions 235 b and the second region 235 c containing the dopant function to relax electric fields (function as “electric field relaxation regions”). The first region 235 a and the pair of second regions 235 b and 235 c containing the dopant compose an oxide semiconductor film 235.

The hydrogen concentration of the first region 235 a in the oxide semiconductor film 235 is preferably lower than 5×10¹⁸ atoms/cm³, more preferably 1×10¹⁸ atoms/cm³ or lower, further preferably 5×10¹⁷ atoms/cm³ or lower, still further preferably 1×10¹⁶ atoms/cm³ or lower. By bonding between an oxide semiconductor and hydrogen, part of contained hydrogen serves as donors to generate electrons as carriers. For that reason, the reduction in the hydrogen concentration of the first region 235 a in the oxide semiconductor film 235 can suppress a negative shift of the threshold voltage.

The dopant concentration of the pair of second regions 235 b and 235 c is higher than or equal to 5×10¹⁸ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³, preferably higher than or equal to 5×10¹⁸ atoms/cm³ and lower than 5×10¹⁹ atoms/cm³.

Since the pair of second regions 235 b and 235 c contains the dopant, the carrier density or the number of defects can be increased. Therefore, the conductivity of the pair of second regions 235 b and 235 c can be higher than that of the first region 235 a which does not contain the dopant. Note that an excessive increase in the dopant concentration causes inhibition of carrier movement by the dopant, which leads to a reduction in conductivity of the pair of second regions 235 b and 235 c containing the dopant.

The pair of second regions 235 b and 235 c containing the dopant preferably has a conductivity of higher than or equal to 0.1 S/cm and lower than or equal to 1000 S/cm, preferably higher than or equal to 10 S/cm and lower than or equal to 1000 S/cm.

The existence of the pair of second regions 235 b and 235 c containing the dopant in the oxide semiconductor film 235 can relieve electric fields applied to an end portion of the first region 235 a serving as the channel region. Thus, a short-channel effect of the transistor can be suppressed.

As a method for adding the dopant to the oxide semiconductor film 229, an ion doping method or an ion implantation method can be used. As the dopant to be added, at least one of boron, nitrogen, phosphorus, and arsenic; at least one of helium, neon, argon, krypton, and xenon; or hydrogen can be used. Alternatively, as the dopant, at least one of boron, nitrogen, phosphorus, and arsenic; at least one of helium, neon, argon, krypton, and xenon; and hydrogen may be used in combination as appropriate.

Here, the dopant is added to the oxide semiconductor film 229 in a state where the oxide semiconductor film 229 is covered with the insulating film and the like; alternatively, the dopant may be added in a state where the oxide semiconductor film 229 is exposed.

Further, the dopant can be added by a method other than injection methods such as ion doping and ion implantation. For example, a dopant can be added in the following manner: plasma is generated in an atmosphere of a gas containing an element to be added and plasma treatment is performed on an object to which the dopant is added. As an apparatus for generating plasma, a dry etching apparatus, a CVD apparatus, a high-density CVD apparatus, or the like can be used.

After that, heat treatment may be performed. The heat treatment is performed typically at a temperature higher than or equal to 150° C. and lower than or equal to 450° C., preferably higher than or equal to 250° C. and lower than or equal to 325° C. In the heat treatment, the temperature may be gradually increased from 250° C. to 325° C.

Through the heat treatment, the resistance of the pair of second regions 235 b and 235 c containing the dopant can be reduced. The heat treatment can be applied to the pair of second regions 235 b and 235 c containing the dopant regardless of their crystalline or amorphous state.

Next, as illustrated in FIG. 8C, sidewall insulating films 237 are formed on side surfaces of the gate electrode 233, and a gate insulating film 239 and electrodes 241 a and 241 b are formed.

The sidewall insulating films 237 are formed with a single layer or a stack of layers using one or more of silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, and the like. Note that like the insulating film 225, the sidewall insulating films 237 may be formed using an oxide insulating film from which part of oxygen is released by heating.

A method for forming the sidewall insulating films 237 is described below.

First, an insulating film to be the sidewall insulating films 237 is formed over the insulating film 231 and the gate electrode 233. The insulating film is formed by sputtering, CVD, or the like. There is no particular limitation on the thickness of the insulating film, and the thickness is selected as appropriate so that the insulating film can cover the gate electrode 233.

Then, part of the insulating film is removed by etching, thereby forming the sidewall insulating films 237. The etching here is highly anisotropic etching, and the sidewall insulating films 237 can be formed in a self-aligned manner by performing the highly anisotropic etching on the insulating film.

The width of the electric field relaxation region in each of the pair of second regions 235 b and 235 c containing the dopant depends on the width of the sidewall insulating film 237, and the width of the sidewall insulating film 237 depends on the thickness of the gate electrode 233. Therefore, the thickness of the gate electrode 233 is determined so that the electric field relaxation region has a desired area.

The gate insulating film 239 can be formed by removing the insulating film 231 by highly anisotropic etching to expose the oxide semiconductor film 235 in the step of forming the sidewall insulating films 237.

The pair of electrodes 241 a and 241 b can be formed using a material similar to that for the wirings 223 a and 223 b as appropriate. Note that the pair of electrodes 241 a and 241 b may function as wirings.

The pair of electrodes 241 a and 241 b is formed by a printing method or an inkjet method. Alternatively, the pair of electrodes 241 a and 241 b is formed in such a manner that a conductive film is formed by sputtering, CVD, evaporation, or the like and then part of the conductive film is selectively etched.

The pair of electrodes 241 a and 241 b is preferably formed to be in contact with side surfaces of the sidewall insulating films 237 and side surfaces of the gate insulating film 239. In other words, it is preferable that end portions of the pair of electrodes 241 a and 241 b of the transistor be located over the sidewall insulating films 237 and entirely cover exposed portions of the pair of second regions 235 b and 235 c containing the dopant in the oxide semiconductor film 235. Accordingly, in the pair of second regions 235 b and 235 c containing the dopant, regions in contact with the pair of electrodes 241 a and 241 b serve as a source region and a drain region, whereas regions overlapping with the sidewall insulating films 237 and the gate insulating film 239 serve as electric field relaxation regions. In addition, since the width of the electric field relaxation region can be controlled with the width of the sidewall insulating film 237, the degree of the accuracy of mask alignment for forming the pair of electrodes 241 a and 241 b can be lowered. Thus, variations between transistors can be reduced.

Although the sidewall insulating films 237 are provided in contact with the side surfaces of the gate electrode 233 in this embodiment, the present invention is not limited to this structure and the sidewall insulating films 237 are not necessarily provided. In addition, the sidewall insulating films 237 are provided after the formation of the pair of second regions 235 b and 235 c in this embodiment; however, the present invention is not limited to this, and the pair of second regions 235 b and 235 c may be formed after the sidewall insulating films 237 are provided. In that case, the area of the first region 235 a can be increased by regions overlapping with the sidewall insulating films 237.

Next, as illustrated in FIG. 9A, an insulating film 243 and an insulating film 245 are formed by sputtering, CVD, coating, printing, or the like.

Each of the insulating films 243 and 245 is formed with a single layer or a stack of layers using one or more of silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, and the like. When an insulating film that prevents diffusion of oxygen to the outside is used as the insulating film 245, oxygen released from the insulating film 243 can be supplied to the oxide semiconductor film. Typical examples of the insulating film that prevents diffusion of oxygen to the outside are an aluminum oxide film and an aluminum oxynitride film. In addition, when an insulating film that prevents diffusion of hydrogen from the outside is used as the insulating film 245, diffusion of hydrogen from the outside to the oxide semiconductor film can be reduced, and oxygen vacancies in the oxide semiconductor film can be reduced. Typical examples of the insulating film that prevents diffusion of hydrogen from the outside are films of silicon nitride, silicon nitride oxide, aluminum nitride, and aluminum nitride oxide. Further, when the insulating film 243 has a three-layer structure in which an oxide insulating film from which part of oxygen is released by heating, an insulating film that prevents diffusion of oxygen to the outside, and an oxide insulating film are stacked, diffusion of oxygen into the oxide semiconductor film can be efficiently performed and release of oxygen to the outside can be suppressed; thus, the change in the characteristics of the transistor can be reduced even at high temperature and high humidity.

Through the above steps, the oxide semiconductor transistor 110 can be fabricated as illustrated in FIG. 9A. Note that the oxide semiconductor transistor 110 includes the oxide semiconductor film 235 which is i-type (intrinsic) or substantially i-type, and therefore has exceptional characteristics.

Note that the transistor 110 has a top-gate structure in this embodiment; however, the present invention is not limited to this and the transistor 110 may have a bottom-gate structure, for example. Further, in the transistor 110 in this embodiment, the pair of electrodes 241 a and 241 b is in contact with at least part of top surfaces of the pair of second regions 235 b and 235 c; however, the present invention is not limited to this structure, and for example, the pair of second regions 235 b and 235 c may be in contact with at least part of the pair of electrodes 241 a and 241 b. Moreover, an impurity region is not necessarily provided in the oxide semiconductor film 229.

Next, parts of the insulating films 215, 217, 221, 225, 243, and 245 are selectively etched to form openings, whereby parts of the gate electrode 209 and the electrodes 241 a and 241 b are exposed. Subsequently, a conductive film is formed in the openings, and then part of the conductive film is selectively etched to form a wiring 249 in contact with the electrode 241 b and a wiring 250 in contact with the electrode 241 a. For the wirings 249 and 250, a material for the contact plugs 219 a and 219 b can be used as appropriate.

Here, the wiring 249 functions as a node that electrically connects one of the source electrode and the drain electrode of the transistor 110 and the gate electrode 209 of the transistor 112. The wiring 250 functions as the other of the source electrode and the drain electrode of the transistor 110 and is electrically connected to the bit line B illustrated in FIG. 1A. Although not directly shown in FIG. 9B, the gate electrode 233 of the transistor 110 is electrically connected to the word line W illustrated in FIG. 1A.

In FIG. 9B, the one of the source electrode and the drain electrode (the electrode 241 b) of the transistor 110 and the gate electrode 209 of the transistor 112 are connected through the wiring 249; however, this embodiment is not limited to this structure. For example, a top surface of the gate electrode of the transistor 112 may be exposed from a top surface of the insulating film provided over the transistor 112, and one of the source electrode and the drain electrode of the transistor 110 may be provided in direct contact with the top surface of the gate electrode.

Through the above steps, the transistors 110 and 112 which serve as a switch of the unit cell can be fabricated.

As has been described, the transistor including a wide bandgap semiconductor, such as an oxide semiconductor, with which the off-state current can be sufficiently low is used in a switch of the unit cell included in the programmable circuit, whereby data for changing the conduction state of the unit cell can be held for a long time even when supply of a power supply potential is stopped. In addition, a plurality of unit cells including analog elements are provided in the programmable circuit and the conduction state of each unit cell is changed between an on state and an off state, so that an analog value in the entire programmable circuit can be made variable.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 4

In this embodiment, the field-effect mobility of the transistor including the oxide semiconductor film described in Embodiment 3 will be derived theoretically, and the transistor characteristics will be derived from the field-effect mobility.

The actually measured field-effect mobility of an insulated gate transistor is lower than its original mobility for a variety of reasons; this phenomenon occurs not only in the case of using an oxide semiconductor. One of the reasons that reduce the mobility is a defect inside a semiconductor or a defect at an interface between the semiconductor and an insulating film. When a Levinson model is used, the field-effect mobility on the assumption that no defect exists inside the semiconductor can be calculated theoretically.

Assuming a potential barrier (such as a grain boundary) exists in a semiconductor, the measured field-effect mobility of the semiconductor, denoted by μ, is expressed by Formula 2 where the inherent field-effect mobility of the semiconductor is μ₀.

$\begin{matrix} {\mu = {\mu_{0}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \end{matrix}$

Here, E denotes the height of the potential barrier, k denotes the Boltzmann constant, and T denotes the absolute temperature. Further, on the assumption that the potential barrier is attributed to a defect, the height of the potential barrier is expressed by Formula 3 according to the Levinson model.

$\begin{matrix} {E = {\frac{e^{2}N^{2}}{8\; ɛ\; n} = \frac{e^{3}N^{2}t}{8\; ɛ\; C_{ox}V_{g}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \end{matrix}$

Here, e represents the elementary charge, N represents the average defect density per unit area in a channel, ∈ represents the dielectric constant of the semiconductor, n represents the number of carriers per unit area in the channel, C_(ox) represents the capacitance per unit area, V_(g) represents the gate voltage, and t represents the thickness of the channel. In the case where the thickness of a semiconductor layer is 30 nm or less, the thickness of the channel can be regarded as being the same as the thickness of the semiconductor layer. The drain current I_(d) in a linear region is expressed by Formula 4.

$\begin{matrix} {I_{d} = {\frac{W\;\mu\; V_{g}V_{d}C_{ox}}{L}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack \end{matrix}$

Here, L represents the channel length and W represents the channel width, and L and W are each 10 μm in this case. In addition, V_(d) represents the drain voltage.

When dividing both sides of the Formula 4 by V_(g) and then taking logarithms of both sides, Formula 5 is obtained.

$\begin{matrix} \begin{matrix} {{\ln\left( \frac{I_{d}}{V_{g}} \right)} = {{\ln\left( \frac{W\;\mu\; V_{d}C_{ox}}{L} \right)} - \frac{E}{kT}}} \\ {= {{\ln\left( \frac{W\;\mu\; V_{d}C_{ox}}{L} \right)} - \frac{e^{3}N^{2}t}{8\mspace{14mu}{kT}\; ɛ\; C_{ox}V_{g}}}} \end{matrix} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack \end{matrix}$

The right side of Formula 5 is a function of V_(g). From Formula 5, it is found that the defect density N can be obtained from a line in a graph that is obtained by plotting actual measured values with ln(I_(d)/V_(g)) as the ordinate and 1/V_(g) as the abscissa. That is, the defect density can be evaluated from the I_(d)−V_(g) characteristics of the transistor. The defect density N of an oxide semiconductor in which the ratio of indium (In), tin (Sn), and zinc (Zn) is 1:1:1 is approximately 1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like, μ₀ can be calculated to be 120 cm²/V·s from Formula 2 and Formula 3. The measured mobility of an In—Sn—Zn oxide having a defect is approximately 40 cm²/V·s. However, assuming that no defect exists inside the semiconductor and at the interface between the semiconductor and an insulating film, the mobility μ₀ of the oxide semiconductor is expected to be 120 cm²/V·s.

Note that even when no defect exists inside a semiconductor, scattering at an interface between a channel and a gate insulator affects the transport property of the transistor. When taking this phenomenon into account, the mobility μ₁ at a position that is distance x away from the interface between the channel and the gate insulator is expressed by Formula 6.

$\begin{matrix} {\frac{1}{\mu_{1}} = {\frac{1}{\mu_{0}} + {\frac{D}{B}{\exp\left( {- \frac{x}{l}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack \end{matrix}$

Here, D represents the electric field in the gate direction, and B and l are constants. The values of B and l can be obtained from actual measurement results; according to the above measurement results, B is 4.75×10⁷ cm/s and l is 10 nm (the depth to which the influence of interface scattering reaches). When D is increased (i.e., when the gate voltage is increased), the second term of Formula 6 is increased and accordingly the mobility μ₁ is decreased.

FIG. 16 shows calculation results of the mobility μ₂ of a transistor whose channel formation region is formed using an ideal oxide semiconductor without a defect inside the semiconductor. For the calculation, device simulation software Sentaurus Device manufactured by Synopsys, Inc. was used, and the bandgap, the electron affinity, the relative permittivity, and the thickness of the oxide semiconductor were assumed to be 2.8 eV, 4.7 eV, 15, and 15 nm, respectively. These values were obtained by measurement of a thin film that was formed by sputtering.

Further, the work functions of a gate, a source, and a drain were assumed to be 5.5 eV, 4.6 eV, and 4.6 eV, respectively. The thickness of a gate insulator was assumed to be 100 nm, and the relative permittivity thereof was assumed to be 4.1. The channel length and the channel width were each assumed to be 10 μm, and the drain voltage V_(d) was assumed to be 0.1 V.

As shown in FIG. 16, the mobility has a peak of more than 100 cm²/V·s at a gate voltage that is a little over 1 V, and is decreased as the gate voltage becomes higher because the influence of interface scattering is increased. Note that in order to reduce interface scattering, it is preferable that a surface of the semiconductor layer be flat at the atomic level (atomic layer flatness).

FIGS. 17A to 17C, FIGS. 18A to 18C, and FIGS. 19A to 19C show calculation results of characteristics of minute transistors formed using an oxide semiconductor having such a mobility. FIGS. 20A and 20B illustrate cross-sectional structures of the transistors used for the calculation. The transistors illustrated in FIGS. 20A and 20B each include a second region 1103 b and a second region 1103 c that have n⁺-type conductivity in an oxide semiconductor layer. The resistivity of the second regions 1103 b and 1103 c is 2×10⁻³ Ω·cm.

The transistor in FIG. 20A is formed over a base insulating film 1101 and an embedded insulator 1102 that is formed using aluminum oxide and embedded in the base insulating film 1101. The transistor includes the second regions 1103 b and 1103 c, an intrinsic first region 1103 a that is positioned between the second regions 1103 b and 1103 c and serves as a channel formation region, and a gate electrode 1105. The width of the gate electrode 1105 is 33 nm.

A gate insulating film 1104 is formed between the gate electrode 1105 and the first region 1103 a. A sidewall insulating film 1106 a and a sidewall insulating film 1106 b are formed on both side surfaces of the gate electrode 1105. An insulator 1107 is formed over the gate electrode 1105 so as to prevent a short circuit between the gate electrode 1105 and another wiring. The sidewall insulating film has a width of 5 nm. A source electrode 1108 a and a drain electrode 1108 b are provided in contact with the second region 1103 b and the second region 1103 c, respectively. Note that the channel width of the transistor is 40 nm.

The transistor in FIG. 20B is the same as the transistor in FIG. 20A in that it is formed over the base insulating film 1101 and the embedded insulator 1102 formed using aluminum oxide and that it includes the second region 1103 b, the second region 1103 c, the intrinsic first region 1103 a provided therebetween, the gate electrode 1105 having a width of 33 nm, the gate insulating film 1104, the sidewall insulating film 1106 a, the sidewall insulating film 1106 b, the insulator 1107, the source electrode 1108 a, and the drain electrode 1108 b.

The transistor in FIG. 20A is different from the transistor in FIG. 20B in the conductivity type of semiconductor regions under the sidewall insulating films 1106 a and 1106 b. In the transistor in FIG. 20A, the semiconductor regions under the sidewall insulating films 1106 a and 1106 b are parts of the second regions 1103 b and 1103 c having n⁺-type conductivity, whereas in the transistor in FIG. 20B, the semiconductor regions under the sidewall insulating films 1106 a and 1106 b are parts of the intrinsic first region 1103 a. In other words, a region having a width of L_(off) which overlaps with neither the second region 1103 b (the second region 1103 c) nor the gate electrode 1105 is provided. This region is called an offset region, and the width L_(off) is called an offset length. As is seen from the drawing, the offset length is equal to the width of the sidewall insulating film 1106 a (the sidewall insulating film 1106 b).

The other parameters used in the calculation are as described above. For the calculation, device simulation software Sentaurus Device manufactured by Synopsys, Inc. was used. FIGS. 17A to 17C show the gate voltage (V_(g): a potential difference between the gate and the source) dependence of the drain current (I_(d), a solid line) and the mobility (μ, a dotted line) of the transistor having the structure illustrated in FIG. 20A. The drain current I_(d) is obtained by calculation under the assumption that the drain voltage (a potential difference between the drain and the source) is +1 V and the mobility μ is obtained by calculation under the assumption that the drain voltage is +0.1 V.

FIG. 17A shows the gate voltage dependence of the transistor in the case where the thickness of the gate insulating film is 15 nm, FIG. 17B shows that of the transistor in the case where the thickness of the gate insulating film is 10 nm, and FIG. 17C shows that of the transistor in the case where the thickness of the gate insulating film is 5 nm. As the gate insulating film is thinner, the drain current I_(d) in the off state (off-state current) in particular is significantly decreased. In contrast, there is no noticeable change in the peak value of the mobility μ and the drain current I_(d) in the on state (on-state current). The graphs show that the drain current exceeds 10 μA, which is required in a memory element and the like, at a gate voltage of around 1 V.

FIGS. 18A to 18C show the gate voltage V_(g) dependence of the drain current I_(d) (a solid line) and the mobility μ (a dotted line) of the transistor having the structure in FIG. 20B and an offset length L_(off) of 5 nm The drain current I_(d) is obtained by calculation under the assumption that the drain voltage is +1 V and the mobility μ is obtained by calculation under the assumption that the drain voltage is +0.1 V. FIG. 18A shows the gate voltage dependence of the transistor in the case where the thickness of the gate insulating film is 15 nm, FIG. 18B shows that of the transistor in the case where the thickness of the gate insulating film is 10 nm, and FIG. 18C shows that of the transistor in the case where the thickness of the gate insulating film is 5 nm.

FIGS. 19A to 19C show the gate voltage dependence of the drain current I_(d) (a solid line) and the mobility μ (a dotted line) of the transistor having the structure in FIG. 20B and an offset length L_(off) of 15 nm The drain current I_(d) is obtained by calculation under the assumption that the drain voltage is +1 V and the mobility μ is obtained by calculation under the assumption that the drain voltage is +0.1 V. FIG. 19A shows the gate voltage dependence of the transistor in the case where the thickness of the gate insulating film is 15 nm, FIG. 19B shows that of the transistor in the case where the thickness of the gate insulating film is 10 nm, and FIG. 19C shows that of the transistor in the case where the thickness of the gate insulating film is 5 nm.

In either of the structures, as the gate insulating film is thinner, the off-state current is significantly decreased, whereas no noticeable change arises in the peak value of the mobility μ and the on-state current.

Note that the peak of the mobility μ is approximately 80 cm²/V·s in FIGS. 17A to 17C, approximately 60 cm²/V·s in FIGS. 18A to 18C, and approximately 40 cm²/V·s in FIGS. 19A to 19C; thus, the peak of the mobility μ is decreased as the offset length L_(off) is increased. Further, the same applies to the off-state current. The on-state current is also decreased as the offset length L_(off) is increased; however, the decrease in the on-state current is much more gradual than the decrease in the off-state current. Further, the graphs show that in either of the structures, the drain current exceeds 10 μA, which is required in a memory element and the like, at a gate voltage of around 1 V. When the transistor with such high mobility is used in a memory unit of the unit cell included in the programmable circuit shown in the foregoing embodiments, data for changing the conduction state of the unit cell can be written at high speed.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 5

In this embodiment, a transistor including the oxide semiconductor film containing In, Sn, and Zn as its main components will be specifically described as an example of the transistor including the oxide semiconductor film shown in Embodiments 3 and 4.

A transistor in which an oxide semiconductor containing In, Sn, and Zn as its main components is used for a channel formation region can have favorable characteristics by depositing the oxide semiconductor while heating a substrate or by performing heat treatment after the oxide semiconductor film is formed. Note that a main component refers to an element included in a composition at 5 atomic % or more.

By intentionally heating the substrate after formation of the oxide semiconductor film containing In, Sn, and Zn as its main components, the field-effect mobility of the transistor can be increased. Further, the threshold voltage of the transistor can be positively shifted to make the transistor normally off. A description is given below of fabrication of a transistor including an oxide semiconductor film containing In, Sn, and Zn as its main components and the results of various measurements of the transistor.

First, the structure of the transistor used for various measurements in this embodiment will be described with reference to FIGS. 26A and 26B. FIG. 26A is a plan view of the transistor, and FIG. 26B is a cross-sectional view along dashed line A-B in FIG. 26A.

The transistor illustrated in FIG. 26B includes a substrate 600, a base insulating film 602 provided over the substrate 600, an oxide semiconductor film 606 provided over the base insulating film 602, a pair of electrodes 614 in contact with the oxide semiconductor film 606, a gate insulating film 608 provided over the oxide semiconductor film 606 and the pair of electrodes 614, a gate electrode 610 provided to overlap with the oxide semiconductor film 606 with the gate insulating film 608 positioned therebetween, an interlayer insulating film 616 provided to cover the gate insulating film 608 and the gate electrode 610, wirings 618 electrically connected to the pair of electrodes 614 through openings formed in the interlayer insulating film 616, and a protective film 620 provided to cover the interlayer insulating film 616 and the wirings 618. The pair of electrodes 614 functions as source and drain electrodes of the transistor.

Here, the substrate 600 was a glass substrate. The base insulating film 602 was a silicon oxide film. The oxide semiconductor film 606 was an In—Sn—Zn—O film. As the pair of electrodes 614, a tungsten film was used. The gate insulating film 608 was a silicon oxide film. The gate electrode 610 had a layered structure of a tantalum nitride film and a tungsten film. The interlayer insulating film 616 had a layered structure of a silicon oxynitride film and a polyimide film. The wirings 618 had a layered structure in which a titanium film, an aluminum film, and a titanium film were formed in this order. The protective film 620 was a polyimide film.

Note that in the transistor having the structure illustrated in FIG. 26A, the width of a portion where the gate electrode 610 overlaps with one of the pair of electrodes 614 is referred to as L_(ov). Similarly, the width of a portion of the pair of electrodes 614, which does not overlap with the oxide semiconductor film 606, is referred to as dW.

FIGS. 21A to 21C each show characteristics of a transistor that has the structure illustrated in FIGS. 26A and 26B and includes an oxide semiconductor film with a channel length L of 3 μm and a channel width W of 10 μm, and a 100-nm-thick gate insulating film. Note that V_(d) was set to 10 V.

FIG. 21A shows characteristics of a transistor whose oxide semiconductor film containing In, Sn, and Zn as its main components was formed by sputtering without heating a substrate intentionally. The field-effect mobility of the transistor is 18.8 cm²/V·s. On the other hand, when the oxide semiconductor film containing In, Sn, and Zn as its main components is formed while the substrate is heated intentionally, the field-effect mobility can be increased. FIG. 21B shows characteristics of a transistor whose oxide semiconductor film containing In, Sn, and Zn as its main components was formed while heating a substrate at 200° C. The field-effect mobility of the transistor is 32.2 cm²/V·s.

The field-effect mobility can be further increased by performing heat treatment after formation of the oxide semiconductor film containing In, Sn, and Zn as its main components. FIG. 21C shows characteristics of a transistor whose oxide semiconductor film containing In, Sn, and Zn as its main components was formed by sputtering at 200° C. and then subjected to heat treatment at 650° C. The field-effect mobility of the transistor is 34.5 cm²/V·s.

The intentional heating of the substrate is expected to have an effect of reducing moisture taken into the oxide semiconductor film during the deposition by sputtering. Further, the heat treatment after the deposition enables hydrogen, a hydroxyl group, or moisture to be released and removed from the oxide semiconductor film. In this manner, the field-effect mobility can be increased. Such an increase in field-effect mobility is presumed to be achieved not only by removal of impurities by dehydration or dehydrogenation but also by a reduction in interatomic distance due to an increase in density. The oxide semiconductor can be crystallized by being highly purified by removal of impurities from the oxide semiconductor. In the case of using such a highly purified non-single-crystal oxide semiconductor, ideally, a field-effect mobility exceeding 100 cm²/V·s is expected to be realized.

The oxide semiconductor containing In, Sn, and Zn as its main components may be crystallized in the following manner: oxygen ions are implanted into the oxide semiconductor, hydrogen, a hydroxyl group, or moisture included in the oxide semiconductor is released by heat treatment, and the oxide semiconductor is crystallized through the heat treatment or by another heat treatment performed later. By such crystallization treatment or recrystallization treatment, a non-single crystal oxide semiconductor having favorable crystallinity can be obtained.

The intentional heating of the substrate during the deposition and/or the heat treatment after the deposition contributes not only to increasing field-effect mobility but also to making the transistor normally off. In a transistor in which an oxide semiconductor film that contains In, Sn, and Zn as its main components and is formed without heating a substrate intentionally is used as a channel formation region, the threshold voltage tends to be shifted negatively. In contrast, when the oxide semiconductor film formed while heating the substrate intentionally is used, the problem of the negative shift of the threshold voltage can be solved. That is, the threshold voltage is shifted so that the transistor becomes normally off; this tendency can be confirmed by comparison between FIGS. 21A and 21B.

Note that the threshold voltage can also be controlled by changing the ratio of In, Sn, and Zn; when the composition ratio of In, Sn, and Zn is 2:1:3, a normally-off transistor is expected to be formed. In addition, an oxide semiconductor film having high crystallinity can be obtained by using a target with a composition ratio of In:Sn:Zn=2:1:3.

The temperature of the intentional heating of the substrate or the temperature of the heat treatment is 150° C. or higher, preferably 200° C. or higher, further preferably 400° C. or higher. When the deposition or heat treatment is performed at a high temperature, the transistor can be normally off.

The heat treatment can be performed in an oxygen atmosphere; alternatively, the heat treatment may be performed first in an atmosphere of nitrogen or an inert gas or under reduced pressure, and then in an atmosphere including oxygen. Oxygen is supplied to the oxide semiconductor after dehydration or dehydrogenation, whereby an effect of the heat treatment can be further enhanced. In order to supply oxygen after dehydration or dehydrogenation, a method in which oxygen ions are accelerated by electric fields and implanted into the oxide semiconductor film may be employed.

A defect due to oxygen vacancy is easily caused in the oxide semiconductor or at an interface between the oxide semiconductor and a stacked film; however, when excess oxygen is included in the oxide semiconductor by the heat treatment, oxygen vacancy caused constantly can be compensated for with excess oxygen. The excess oxygen is oxygen existing mainly between lattices. When the concentration of oxygen is set in the range of 1×10¹⁶/cm³ to 2×10²⁰/cm³, excess oxygen can be included in the oxide semiconductor without causing crystal distortion or the like.

When heat treatment is performed so that at least part of the oxide semiconductor includes crystal, a more stable oxide semiconductor film can be obtained. For example, when an oxide semiconductor film that is formed by sputtering using a target having a composition ratio of In:Sn:Zn=1:1:1 without heating a substrate intentionally is analyzed by X-ray diffraction (XRD), a halo pattern is observed. The formed oxide semiconductor film can be crystallized by being subjected to heat treatment. The temperature of the heat treatment can be set as appropriate; when the heat treatment is performed at 650° C., for example, a clear diffraction peak can be observed with X-ray diffraction.

An XRD analysis of an In—Sn—Zn—O film was conducted. The XRD analysis was conducted using an X-ray diffractometer D8 ADVANCE manufactured by Bruker AXS, and measurement was performed by an out-of-plane method.

Sample A and Sample B were prepared and the XRD analysis was performed thereon. Methods for manufacturing Sample A and Sample B will be described below.

An In—Sn—Zn—O film with a thickness of 100 nm was formed over a quartz substrate that had been subjected to dehydrogenation treatment.

The In—Sn—Zn—O film was formed with a sputtering apparatus with a power of 100 W (DC) in an oxygen atmosphere. An In—Sn—Zn—O target having an atomic ratio of In:Sn:Zn=1:1:1 was used as a target. Note that the substrate heating temperature in film formation was 200° C. A sample manufactured in this manner was used as Sample A.

Next, a sample manufactured by a method similar to that of Sample A was subjected to heat treatment at 650° C. As the heat treatment, heat treatment in a nitrogen atmosphere was first performed for one hour and then heat treatment in an oxygen atmosphere was performed for one hour without lowering the temperature. A sample manufactured in this manner was used as Sample B.

FIG. 22 shows XRD spectra of Sample A and Sample B. No peak derived from crystal was observed in Sample A, whereas peaks derived from crystal were observed when 20 was around 35 deg and 37 deg to 38 deg in Sample B.

As described above, by intentionally heating a substrate during deposition of an oxide semiconductor containing In, Sn, and Zn as its main components and/or by performing heat treatment after the deposition, characteristics of a transistor can be improved.

These substrate heating and heat treatment have an effect of preventing hydrogen and a hydroxyl group, which are unfavorable impurities for an oxide semiconductor, from being included in the film or an effect of removing hydrogen and a hydroxyl group from the film. That is, an oxide semiconductor can be highly purified by removing hydrogen serving as a donor impurity from the oxide semiconductor, whereby a normally-off transistor can be obtained. The high purification of the oxide semiconductor enables the off-state current of the transistor to be 1 aA/μm or lower. Here, the unit of the off-state current represents current per micrometer of a channel width.

FIG. 23 shows a relation between the off-state current of a transistor and the inverse of substrate temperature (absolute temperature) at measurement. Here, for simplicity, the horizontal axis represents a value (1000/T) obtained by multiplying an inverse of the substrate temperature at measurement by 1000.

Specifically, as shown in FIG. 22, the off-state current was 0.1 aA/μm (1×10⁻¹⁹ A/μm) or smaller, and 10 zA/μm (1×10⁻²⁰ A/μm) or smaller when the substrate temperature was 125° C. and 85° C., respectively. The proportional relation between the logarithm of the off-state current and the inverse of the temperature suggests that the off-state current at room temperature (27° C.) is 0.1 zA/μm (1×10⁻²² A/μm) or smaller. Hence, the off-state current can be 1 aA/μm (1×10⁻¹⁸ A/μm) or smaller, 100 zA/μm (1×10⁻¹⁹ A/μm) or smaller, and 1 zA/μm (1×10⁻²¹ A/μm) or smaller at 125° C., 85° C., and room temperature, respectively.

Note that in order to prevent hydrogen and moisture from being included in the oxide semiconductor film during the deposition, it is preferable to increase the purity of a sputtering gas by sufficiently suppressing leakage from the outside of a deposition chamber and degasification through an inner wall of the deposition chamber. For example, a gas with a dew point of −70° C. or lower is preferably used as the sputtering gas in order to prevent moisture from being included in the film. In addition, it is preferable to use a target which is highly purified so as not to include impurities such as hydrogen and moisture. Although it is possible to remove moisture from a film of an oxide semiconductor containing In, Sn, and Zn as its main components by heat treatment, a film that does not include moisture originally is preferably formed because moisture is released from the oxide semiconductor containing In, Sn, and Zn as its main components at a higher temperature than from an oxide semiconductor containing In, Ga, and Zn as its main components.

The relation between the substrate temperature and electric characteristics of a transistor of Sample B, on which heat treatment at 650° C. was performed after formation of the oxide semiconductor film, was evaluated.

The transistor used for the measurement has a channel length L of 3 μm, a channel width W of 10 μm, L_(ov) of 3 μm on one side (total L_(ov) of 6 μm), and dW of 0 μm. Note that V_(d) was set to 10 V. The substrate temperature was −40° C., −25° C., 25° C., 75° C., 125° C., and 150° C. Here, in the transistor, the width of a portion where a gate electrode overlaps with one of a pair of electrodes is referred to as L_(ov), and the width of a portion of the pair of electrodes, which does not overlap with an oxide semiconductor film, is referred to as dW.

FIG. 24 shows the V_(g) dependence of I_(d) (a solid line) and field-effect mobility (a dotted line). FIG. 25A shows a relation between the substrate temperature and the threshold voltage, and FIG. 25B shows a relation between the substrate temperature and the field-effect mobility.

From FIG. 25A, it is found that the threshold voltage gets lower as the substrate temperature increases. Note that the threshold voltage is decreased from 1.09 V to −0.23 V in the range from −40° C. to 150° C.

From FIG. 25B, it is found that the field-effect mobility gets lower as the substrate temperature increases. Note that the field-effect mobility is decreased from 36 cm²/V·s to 32 cm²/V·s in the range from −40° C. to 150° C. Thus, it is found that variation in electric characteristics is small in the above temperature range.

In the transistor including the oxide semiconductor layer containing In, Sn, and Zn as its main components, the field-effect mobility can be 30 cm²/V·s or higher, preferably 40 cm²/V·s or higher, further preferably 60 cm²/V·s or higher with the off-state current maintained at 1 aA/μm or lower, and can have on-state current high enough to meet the specifications demanded for LSIs. For example, in an FET with a channel width of 40 nm and a channel length of 33 nm, an on-state current of 12 μA or higher can flow when the gate voltage is 2.7 V and the drain voltage is 1.0 V.

When the transistor with such low off-state current is used in a memory unit of the programmable switch shown in the foregoing embodiments, configuration data can be held even when supply of a power supply potential is stopped. Thus, write of configuration data after power on can be omitted, so that the start time of the logic unit can be shortened. Accordingly, a programmable logic device in which power consumption can be reduced by employing a normally-off driving method can be provided.

In addition, when the transistor with such low off-state current is used in a memory unit of the programmable switch shown in the foregoing embodiments, configuration data can be written at high speed, so that a programmable logic device in which dynamic reconfiguration can be easily achieved can be provided.

Since the transistor including the oxide semiconductor has such characteristics, it is possible to provide a programmable logic device whose operation speed is not decreased even when the transistor including the oxide semiconductor is included in an integrated circuit formed using a silicon semiconductor.

Embodiment 6

With the use of a programmable circuit according to an embodiment of the present invention, a low-power electronic device can be provided. In particular, when a low-power semiconductor device according to an embodiment of the present invention is added as a component of a portable electronic device which has difficulty in continuously receiving power, the portable electronic device can have a long continuous operation time.

A semiconductor device including a programmable circuit according to an embodiment of the present invention can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices that reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other examples of electronic devices that can include a semiconductor device according to an embodiment of the present invention are mobile phones, game machines including portable game machines, personal digital assistants, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), and vending machines.

A description is given of the case where the semiconductor device including the programmable circuit according to an embodiment of the present invention is applied to a portable electronic device such as a mobile phone, a smartphone, or an e-book reader.

FIG. 10 is a block diagram of a portable electronic device. The portable electronic device in FIG. 10 includes an RF circuit 421, an analog baseband circuit 422, a digital baseband circuit 423, a battery 424, a power supply circuit 425, an application processor 426, a flash memory 430, a display controller 431, a memory circuit 432, a display 433, a touch sensor 439, an audio circuit 437, a keyboard 438, and the like. The display 433 includes a display portion 434, a source driver 435, and a gate driver 436. The application processor 426 includes a CPU 427, a DSP 428, and an interface 429. For example, when the programmable circuit described in any of the above embodiments is used for any or all of the RF circuit 421, the analog baseband circuit 422, the memory circuit 432, the application processor 426, the display controller 431, and the audio circuit 437, power consumption can be reduced.

FIG. 11 is a block diagram of an e-book reader. The e-book reader includes a battery 451, a power supply circuit 452, a microprocessor 453, a flash memory 454, an audio circuit 455, a keyboard 456, a memory circuit 457, a touch panel 458, a display 459, and a display controller 460. The microprocessor 453 includes a CPU 461, a DSP 462, and an interface 463. For example, when the programmable circuit described in any of the above embodiments is used for any or all of the audio circuit 455, the memory circuit 457, the microprocessor 453, and the display controller 460, power consumption can be reduced.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

This application is based on Japanese Patent Application serial no. 2011-113180 filed with Japan Patent Office on May 20, 2011, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising a programmable circuit, the programmable circuit comprising: a unit cell comprising: an input signal line, an output signal line; an analog element; a first transistor comprising an oxide semiconductor in a channel formation region, a first source electrode, a first drain electrode, and a first gate electrode; and a second transistor comprising a second source electrode, a second drain electrode, and a second gate electrode, the second gate electrode being electrically connected to one of the first source electrode and the first drain electrode, and one of the second source electrode and the second drain electrode being electrically connected to the analog element, wherein the analog element and the second transistor are connected in series between the input signal line and the output signal line.
 2. The semiconductor device according to claim 1, further comprising additional unit cells each having a same structure as the unit cell, and connected in a matrix including the unit cell.
 3. A semiconductor device comprising a programmable circuit, the programmable circuit including: an input signal line, an output signal line; a column of unit cells comprising unit cells, each of the unit cells comprising: an analog element; a first transistor comprising an oxide semiconductor in a channel formation region, a first source electrode, a first drain electrode, and a first gate electrode; and a second transistor comprising a second source electrode, a second drain electrode, and a second gate electrode, the second gate electrode being electrically connected to one of the first source electrode and the first drain electrode, and one of the second source electrode and the second drain electrode being electrically connected to the analog element, wherein, for each unit cell, the analog element and the second transistor are connected in series between the input signal line and the output signal line.
 4. The semiconductor device according to claim 3, further comprising at least one additional column of unit cells, wherein the outputs of the unit cells of the column are electrically connected to inputs of the unit cells of the at least one additional column of unit cells.
 5. A semiconductor device comprising a programmable circuit, the programmable circuit including: an input signal line, an output signal line; a row of unit cells comprising at least a first unit cell and a second unit cell, each of the unit cells comprising: an analog element; a first transistor comprising an oxide semiconductor in a channel formation region, a first source electrode, a first drain electrode, and a first gate electrode; and a second transistor comprising a second source electrode, a second drain electrode, and a second gate electrode, the second gate electrode being electrically connected to one of the first source electrode and the first drain electrode, and one of the second source electrode and the second drain electrode being electrically connected to the analog element, wherein the analog elements and the second transistors of the unit cells are connected in series between the input signal line and the output signal line.
 6. The semiconductor device according to claim 5, further comprising at least one additional row of unit cells, wherein the analog elements and the second transistors of the unit cells of the additional row of unit cells are connected in series between the input signal line and the output signal line.
 7. The semiconductor device according to claim 5, further comprising at least one additional row of unit cells, wherein the analog elements and the second transistors of the unit cells of the additional row of unit cells are connected in series between the input signal line and the output signal line, and wherein a node between two consecutive unit cells of the row of unit cells is electrically connected to a node between two consecutive unit cells of the additional row of unit cells.
 8. The semiconductor device according to claim 1, wherein the analog element comprises an oxide semiconductor.
 9. The semiconductor device according to claim 3, wherein the analog element comprises an oxide semiconductor.
 10. The semiconductor device according to claim 5, wherein the analog element comprises an oxide semiconductor.
 11. The semiconductor device according to claim 1, wherein the analog element is a capacitor, a resistor, or a photoelectric conversion element.
 12. The semiconductor device according to claim 3, wherein the analog element is a capacitor, a resistor, or a photoelectric conversion element.
 13. The semiconductor device according to claim 5, wherein the analog element is a capacitor, a resistor, or a photoelectric conversion element.
 14. The semiconductor device according to claim 1, each unit cell further comprising: an inverter; a third transistor comprising an oxide semiconductor in a channel formation region, a third source electrode, a third drain electrode, and a third gate electrode, the third gate electrode being electrically connected to the first gate electrode; and a fourth transistor of a type, n-channel or p-channel, different from a type of the second transistor, and comprising a fourth source electrode, a fourth drain electrode, and a fourth gate electrode, wherein the fourth gate electrode is electrically connected to the other of the first source electrode and the first drain electrode through the inverter, the third source electrode, and the fourth source electrode, wherein one of the fourth source electrode and the fourth drain electrode is electrically connected to the one of the second source electrode and the second drain electrode, and wherein the other of the fourth source electrode and the fourth drain electrode being electrically connected to the other of the second source electrode and the second drain electrode.
 15. The semiconductor device according to claim 3, each unit cell further comprising; an inverter; a third transistor comprising an oxide semiconductor in a channel formation region, a third source electrode, a third drain electrode, and a third gate electrode, the third gate electrode being electrically connected to the first gate electrode; and a fourth transistor of a type, n-channel or p-channel, different from a type of the second transistor, and comprising a fourth source electrode, a fourth drain electrode, and a fourth gate electrode, wherein the fourth gate electrode is electrically connected to the other of the first source electrode and the first drain electrode through the inverter, the third source electrode, and the fourth source electrode, wherein one of the fourth source electrode and the fourth drain electrode is electrically connected to the one of the second source electrode and the second drain electrode, and wherein the other of the fourth source electrode and the fourth drain electrode being electrically connected to the other of the second source electrode and the second drain electrode.
 16. The semiconductor device according to claim 5, each unit cell further comprising: an inverter; a third transistor comprising an oxide semiconductor in a channel formation region, a third source electrode, a third drain electrode, and a third gate electrode, the third gate electrode being electrically connected to the first gate electrode; and a fourth transistor of a type, n-channel or p-channel, different from a type of the second transistor, and comprising a fourth source electrode, a fourth drain electrode, and a fourth gate electrode, wherein the fourth gate electrode is electrically connected to the other of the first source electrode and the first drain electrode through the inverter, the third source electrode, and the fourth source electrode, wherein one of the fourth source electrode and the fourth drain electrode is electrically connected to the one of the second source electrode and the second drain electrode, and wherein the other of the fourth source electrode and the fourth drain electrode being electrically connected to the other of the second source electrode and the second drain electrode.
 17. The semiconductor device according to claim 1, wherein the first transistor and the second transistor are formed over a same substrate.
 18. The semiconductor device according to claim 3, wherein the first transistor and the second transistor are formed over a same substrate.
 19. The semiconductor device according to claim 5, wherein the first transistor and the second transistor are formed over a same substrate.
 20. The semiconductor according to claim 1, wherein the unit cell is a programmable cell in which a programmed state is a potential of the second gate electrode kept in a wiring connecting the second gate electrode to the one of the first source electrode and the first drain electrode when the first transistor is turned off.
 21. The semiconductor according to claim 3, wherein each unit cell is a programmable cell in which a programmed state is a potential of the corresponding second gate electrode kept in a wiring connecting the corresponding second gate electrode to the one of the first source electrode and the first drain electrode of the corresponding first transistor when the corresponding first transistor is turned off.
 22. The semiconductor according to claim 5, wherein each unit cell is a programmable cell in which a programmed state is a potential of the corresponding second gate electrode kept in a wiring connecting the corresponding second gate electrode to the one of the first source electrode and the first drain electrode of the corresponding first transistor when the corresponding first transistor is turned off.
 23. The semiconductor device according to claim 1, wherein the semiconductor device is one of an RF circuit, an analog baseband circuit, a memory circuit, an application processor, a display controller and an audio circuit.
 24. The semiconductor device according to claim 3, wherein the semiconductor device is one of an RF circuit, an analog baseband circuit, a memory circuit, an application processor, a display controller and an audio circuit.
 25. The semiconductor device according to claim 5, wherein the semiconductor device is one of an RF circuit, an analog baseband circuit, a memory circuit, an application processor, a display controller and an audio circuit.
 26. The semiconductor device according to claim 3, wherein inputs of the unit cells are directly connected to the input signal line, and wherein outputs of the units cells are directly connected to the output signal line.
 27. The semiconductor device according to claim 5, wherein an output of the first unit cell is directly connected to an input of the second unit cell. 